add net-diag to help message

cmd/ipfs/ipfs.go

@@ -42,6 +42,8 @@ Advanced Commands:
     mount         Mount an ipfs read-only mountpoint.
     serve         Serve an interface to ipfs.
 
+	net-diag	  Print network diagnostic
+
 Use "ipfs help <command>" for more information about a command.
 `,
 	Run: ipfsCmd,