docs: filesystems: vfs: correct flag name

There is no flag REMAP_CAN_SHORTEN.  Commit eca3654 ("vfs: enable
remap callers that can handle short operations") that introduces this
text also introduces the flag REMAP_FILE_CAN_SHORTEN.  Change the name
in the documentation accordingly.

Signed-off-by: Julia Lawall <Julia.Lawall@inria.fr>
Fixes: eca3654 ("vfs: enable remap callers that can handle short operations")
Link: https://lore.kernel.org/r/1595789020-12941-1-git-send-email-Julia.Lawall@inria.fr
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/filesystems/vfs.rst

@@ -1116,7 +1116,7 @@ otherwise noted.
 	before any bytes were remapped.  The remap_flags parameter
 	accepts REMAP_FILE_* flags.  If REMAP_FILE_DEDUP is set then the
 	implementation must only remap if the requested file ranges have
-	identical contents.  If REMAP_CAN_SHORTEN is set, the caller is
+	identical contents.  If REMAP_FILE_CAN_SHORTEN is set, the caller is
 	ok with the implementation shortening the request length to
 	satisfy alignment or EOF requirements (or any other reason).