cifsd: fix WARNING: Title overline too short

Stephen reported a warning message from cifsd.rst file.

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

Documentation/filesystems/cifs/cifsd.rst

 .. SPDX-License-Identifier: GPL-2.0
 
-=========================
+==========================
 CIFSD - SMB3 Kernel Server
-=========================
+==========================
 
 CIFSD is a linux kernel server which implements SMB3 protocol in kernel space
 for sharing files over network.