doc: update fs.open() changes record for optional 'flags'

Was missed on original PR.

Ref: #23767

PR-URL: #24240
Refs: #23767
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/fs.md

@@ -2308,6 +2308,9 @@ object with an `encoding` property specifying the character encoding to use.
 <!-- YAML
 added: v0.0.2
 changes:
+  - version: v11.1.0
+    pr-url: https://github.com/nodejs/node/pull/23767
+    description: The `flags` argument is now optional and defaults to `'r'`.
   - version: v9.9.0
     pr-url: https://github.com/nodejs/node/pull/18801
     description: The `as` and `as+` modes are supported now.
@@ -2344,6 +2347,12 @@ Functions based on `fs.open()` exhibit this behavior as well:
 <!-- YAML
 added: v0.1.21
 changes:
+  - version: v11.1.0
+    pr-url: https://github.com/nodejs/node/pull/23767
+    description: The `flags` argument is now optional and defaults to `'r'`.
+  - version: v9.9.0
+    pr-url: https://github.com/nodejs/node/pull/18801
+    description: The `as` and `as+` modes are supported now.
   - version: v7.6.0
     pr-url: https://github.com/nodejs/node/pull/10739
     description: The `path` parameter can be a WHATWG `URL` object using `file:`
@@ -4204,10 +4213,15 @@ characters directly to the `prefix` string. For instance, given a directory
 ### fsPromises.open(path, flags[, mode])
 <!-- YAML
 added: v10.0.0
+changes:
+  - version: v11.1.0
+    pr-url: https://github.com/nodejs/node/pull/23767
+    description: The `flags` argument is now optional and defaults to `'r'`.
 -->
 
 * `path` {string|Buffer|URL}
 * `flags` {string|number} See [support of file system `flags`][].
+  **Default:** `'r'`.
 * `mode` {integer} **Default:** `0o666` (readable and writable)
 * Returns: {Promise}