fix typo for server create in server.rst

we have <security-group-list> in server create option,
while <security-group-name> in the explanation, they
both should be <security-group-name>.

Closes-Bug: #1466742
Change-Id: I26a25f57e57d8f9e19ec9c9ccb3c2b8d5396b78d

Author: Guojian Shao

doc/source/command-objects/server.rst

@@ -60,7 +60,7 @@ Create a new server
     os server create
         --image <image> | --volume <volume>
         --flavor <flavor>
-        [--security-group <security-group-list> [...] ]
+        [--security-group <security-group-name> [...] ]
         [--key-name <key-name>]
         [--property <key=value> [...] ]
         [--file <dest-filename=source-filename>] [...] ]