cifs: fix typo in module parameter enable_gcm_256

enable_gcm_256 (which allows the server to require the strongest
encryption) is enabled by default, but the modinfo description
incorrectly showed it disabled by default. Fix the typo.

Cc: stable@vger.kernel.org
Fixes: fee742b5 ("smb3.1.1: enable negotiating stronger encryption by default")
Signed-off-by: Steve French <stfrench@microsoft.com>