Fix server group name on api-ref

The parameter 'name' is for security group, not server group.
This patch fixes the description.

Partially implements blueprint api-ref-in-rst

Change-Id: I954b91c47bf4766be430ace2783089c7f59f9324

api-ref/source/os-server-groups.inc

@@ -61,7 +61,7 @@ Request
 
 .. rest_parameters:: parameters.yaml
 
-  - name: name
+  - name: name_server_group
   - policies: policies
 
 **Example Create Server Group: JSON request**