doc: use AIX instead of Aix in fs.md

Both for consistency within the doc and to conform with IBM's usage
regarding their on product, refer to the operating system as AIX rather
than Aix.

PR-URL: #38535
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: Beth Griggs <bgriggs@redhat.com>

doc/api/fs.md

@@ -4158,7 +4158,7 @@ to be notified of filesystem changes.
   directories.
 * On SunOS systems (including Solaris and SmartOS), this uses [`event ports`][].
 * On Windows systems, this feature depends on [`ReadDirectoryChangesW`][].
-* On Aix systems, this feature depends on [`AHAFS`][], which must be enabled.
+* On AIX systems, this feature depends on [`AHAFS`][], which must be enabled.
 * On IBM i systems, this feature is not supported.
 
 If the underlying functionality is not available for some reason, then