doc: fix header level in fs.md

Indentation for #fs_fs_promises_api is level 2 but it should be level 3.

PR-URL: #35771
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>

Author: ax1

doc/api/fs.md

@@ -5565,7 +5565,7 @@ that represent files will be deleted. The permissive behavior of the
 `recursive` option is deprecated, `ENOTDIR` and `ENOENT` will be thrown in
 the future.
 
-## `fsPromises.rm(path[, options])`
+### `fsPromises.rm(path[, options])`
 <!-- YAML
 added: v14.14.0
 -->