Stephen reported a warning message from cifsd.rst file.

 Documentation/filesystems/cifs/cifsd.rst:3: WARNING: Title overline too
 short.
Reported-by: NStephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: NNamjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: NSteve French <stfrench@microsoft.com>