doc: cifsd: change the reference to configuration.txt

added documentation for cifsd. There, it points to a file
named:
        Documentation/configuration.txt

This confuses Kernel scripts, as they think that this is a
document within the Kernel tree, instead of a file from
some other place.

Replace it by an hyperlink to the ksmbd-tools tree, in order
to avoid false-positives.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: Steve French <stfrench@microsoft.com>