Mention Samba in intro.rst

Clarify sentence. As mentioned in the previous sentence, Manila
supports both NFS and Samba, so let's mention it here as well.

Change-Id: I3b26d18860dd1919394e704d825e7bece38fc997
Co-Authored-By: David Disseldorp <ddiss@suse.de>
Merge "Mention Samba in intro.rst"