peer network login usage message is wrong

The `peer network login` command does not specify that it
requires a username argument:

$ peer network login
Error: Must supply username
Usage:
  peer network login [flags]

This change modifies the usage message to:

Usage:
  peer network login <username> [flags]

Change-Id: I7ffbc312a9ee33ba34584c4cb57d091ffeb4b2dc
Signed-off-by: Bradley Gorman <bgorman@au1.ibm.com>

peer/network/login.go

@@ -42,7 +42,7 @@ func loginCmd() *cobra.Command {
 }
 
 var networkLoginCmd = &cobra.Command{
-	Use:   "login",
+	Use:   "login <username>",
 	Short: "Logs in user to CLI.",
 	Long:  `Logs in the local user to CLI. Must supply username as a parameter.`,
 	RunE: func(cmd *cobra.Command, args []string) error {