docs: fs: porting.rst: fix a broken reference to another doc

With all those document shifts, references to documents get
broken.

Fix one such occurrence at porting.rst.
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/filesystems/porting.rst

@@ -158,7 +158,7 @@ Callers of notify_change() need ->i_mutex now.
 New super_block field ``struct export_operations *s_export_op`` for
 explicit support for exporting, e.g. via NFS.  The structure is fully
 documented at its declaration in include/linux/fs.h, and in
-Documentation/filesystems/nfs/Exporting.
+Documentation/filesystems/nfs/exporting.rst.
 
 Briefly it allows for the definition of decode_fh and encode_fh operations
 to encode and decode filehandles, and allows the filesystem to use