doc: fix parameter type format

PR-URL: nodejs#19957
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Yuta Hiroto <hello@hiroppy.me>

doc/api/child_process.md

@@ -1091,7 +1091,7 @@ changes:
 * `options` {Object} The `options` argument, if present, is an object used to
   parameterize the sending of certain types of handles. `options` supports
   the following properties:
-  * `keepOpen` - A Boolean value that can be used when passing instances of
+  * `keepOpen` {boolean} A value that can be used when passing instances of
     `net.Socket`. When `true`, the socket is kept open in the sending process.
     **Default:** `false`.
 * `callback` {Function}