docs: config.md: fix typo

Add a missing parenthesis.

Signed-off-by: David Wronek <david@mainlining.org>

docs/config.md

@@ -1379,7 +1379,7 @@ variable before running the `ntfy` command (e.g. `export NTFY_LISTEN_HTTP=:80`).
 | `listen-unix-mode`                         | `NTFY_LISTEN_UNIX_MODE`                         | *file mode*                                         | *system default*  | File mode of the Unix socket, e.g. 0700 or 0777                                                                                                                                                                                 |
 | `key-file`                                 | `NTFY_KEY_FILE`                                 | *filename*                                          | -                 | HTTPS/TLS private key file, only used if `listen-https` is set.                                                                                                                                                                 |
 | `cert-file`                                | `NTFY_CERT_FILE`                                | *filename*                                          | -                 | HTTPS/TLS certificate file, only used if `listen-https` is set.                                                                                                                                                                 |
-| `firebase-key-file`                        | `NTFY_FIREBASE_KEY_FILE`                        | *filename*                                          | -                 | If set, also publish messages to a Firebase Cloud Messaging (FCM) topic for your app. This is optional and only required to save battery when using the Android app. See [Firebase (FCM](#firebase-fcm).                        |
+| `firebase-key-file`                        | `NTFY_FIREBASE_KEY_FILE`                        | *filename*                                          | -                 | If set, also publish messages to a Firebase Cloud Messaging (FCM) topic for your app. This is optional and only required to save battery when using the Android app. See [Firebase (FCM)](#firebase-fcm).                        |
 | `cache-file`                               | `NTFY_CACHE_FILE`                               | *filename*                                          | -                 | If set, messages are cached in a local SQLite database instead of only in-memory. This allows for service restarts without losing messages in support of the since= parameter. See [message cache](#message-cache).             |
 | `cache-duration`                           | `NTFY_CACHE_DURATION`                           | *duration*                                          | 12h               | Duration for which messages will be buffered before they are deleted. This is required to support the `since=...` and `poll=1` parameter. Set this to `0` to disable the cache entirely.                                        |
 | `cache-startup-queries`                    | `NTFY_CACHE_STARTUP_QUERIES`                    | *string (SQL queries)*                              | -                 | SQL queries to run during database startup; this is useful for tuning and [enabling WAL mode](#wal-for-message-cache)                                                                                                           |