etcdctl: remove extra space in error message

Fprintln will insert a space between arguments, so printing "Error: "
would lead to two spaces between the "Error:" and the error string.

etcdctl/ctlv3/command/error.go

@@ -34,7 +34,7 @@ const (
 )
 
 func ExitWithError(code int, err error) {
-	fmt.Fprintln(os.Stderr, "Error: ", err)
+	fmt.Fprintln(os.Stderr, "Error:", err)
 	if cerr, ok := err.(*client.ClusterError); ok {
 		fmt.Fprintln(os.Stderr, cerr.Detail())
 	}