Fix "Tags" section capitalisation in the man pages

"Tags" is a section, not a command, so let's title-case it (as opposed
to all upper-case). Furthermore, the RST markup was wrong for the
gnt-network man page.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>