Fix server group document issue

The patch fix server group document typo and format issue.

Implements: blueprint nova-server-group-support
Related-Bug: #1542171

Change-Id: I3e24ddce91ecabc6be1b7be95d0f4e5a2ebe93ab

Author: kiwik

doc/source/command-objects/server-group.rst

@@ -2,7 +2,7 @@
 server group
 ============
 
-Server group provide a mechanism to group servers according to certain policy.
+Server group provides a mechanism to group servers according to certain policy.
 
 Compute v2
 
@@ -77,4 +77,4 @@ Display server group details
 
 .. describe:: <server-group>
 
-        Server group to display (name or ID)
+    Server group to display (name or ID)