docs: filesystems: vfs: correct flag name

There is no flag REMAP_CAN_SHORTEN.  Commit eca3654e ("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: eca3654e ("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>