doc: standardize preposition usage in fs.md

"On <operating system>" is used everywhere in this file except for one
use of "In <operating system>". Standardize to "On".

PR-URL: #17463
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

Author: Trott

doc/api/fs.md

@@ -2491,7 +2491,7 @@ a new inode. The watch will emit an event for the delete but will continue
 watching the *original* inode. Events for the new inode will not be emitted.
 This is expected behavior.
 
-In AIX, save and close of a file being watched causes two notifications -
+On AIX, save and close of a file being watched causes two notifications -
 one for adding new content, and one for truncation. Moreover, save and
 close operations on some platforms cause inode changes that force watch
 operations to become invalid and ineffective. AIX retains inode for the