Trivial: Do not use plural format in command parameter in "network delete"

Since "network delete" could delete more than one network,
add a (s) to the doc.

And also, rename the parameter "networks" to "network".
The naming style is not using plural format in parameter.

Change-Id: Id434ea905af34457f84ea6bcb18addef5800429a

doc/source/command-objects/network.rst

@@ -62,7 +62,7 @@ Delete network(s)
 .. _network_delete-project:
 .. describe:: <network>
 
-    Network to delete (name or ID)
+    Network(s) to delete (name or ID)
 
 network list
 ------------

openstackclient/network/v2/network.py

@@ -119,17 +119,17 @@ class DeleteNetwork(command.Command):
     def get_parser(self, prog_name):
         parser = super(DeleteNetwork, self).get_parser(prog_name)
         parser.add_argument(
-            'networks',
+            'network',
             metavar="<network>",
             nargs="+",
-            help=("Network to delete (name or ID)")
+            help=("Network(s) to delete (name or ID)")
         )
         return parser
 
     def take_action(self, parsed_args):
         self.log.debug('take_action(%s)' % parsed_args)
         client = self.app.client_manager.network
-        for network in parsed_args.networks:
+        for network in parsed_args.network:
             obj = client.find_network(network)
             client.delete_network(obj)
         return

openstackclient/tests/network/v2/test_network.py

@@ -284,7 +284,7 @@ class TestDeleteNetwork(TestNetwork):
             self._network.name,
         ]
         verifylist = [
-            ('networks', [self._network.name]),
+            ('network', [self._network.name]),
         ]
 
         parsed_args = self.check_parser(self.cmd, arglist, verifylist)