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>

man/gnt-group.rst

@@ -210,7 +210,7 @@ Example::
     # gnt-group evacuate -I hail --to rack4 rack1
 
 
-TAGS
+Tags
 ~~~~
 
 ADD-TAGS

man/gnt-instance.rst

@@ -1685,7 +1685,7 @@ Example::
     # gnt-instance change-group -I hail --to rack2 inst1.example.com
 
 
-TAGS
+Tags
 ~~~~
 
 ADD-TAGS

man/gnt-network.rst

@@ -144,8 +144,8 @@ Disconnect a network from given node groups (all if not specified). This
 is possible only if no instance is using the network.
 
 
-TAGS
-~~~
+Tags
+~~~~
 
 ADD-TAGS
 ^^^^^^^^