docs: filesystems: update netfs-api.rst reference

Changeset efc930fa ("docs: filesystems: caching/netfs-api.txt: convert it to ReST")
renamed: Documentation/filesystems/caching/netfs-api.txt
to: Documentation/filesystems/caching/netfs-api.rst.

Update its cross-reference accordingly.

Fixes: efc930fa ("docs: filesystems: caching/netfs-api.txt: convert it to ReST")
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>
Link: https://lore.kernel.org/r/5f867f01d42c3e65e111167739ed1a41a26623f9.1656234456.git.mchehab@kernel.orgSigned-off-by: Jonathan Corbet <corbet@lwn.net>

include/linux/fscache.h

@@ -378,7 +378,7 @@ void fscache_update_cookie(struct fscache_cookie *cookie, const void *aux_data,
  *
  * Request that the size of an object be changed.
  *
- * See Documentation/filesystems/caching/netfs-api.txt for a complete
+ * See Documentation/filesystems/caching/netfs-api.rst for a complete
  * description.
  */
 static inline