fix upgrade function prompt (#287)

Author: zhangyangyu

internal/cli/root.go

@@ -187,7 +187,7 @@ func RootCmd(h *internal.Helper) *cobra.Command {
 					if config.IsUnderTiUP {
 						fmt.Fprintln(h.IOStreams.Out, color.GreenString("Use `tiup update cloud` to update to the latest version"))
 					} else {
-						fmt.Fprintln(h.IOStreams.Out, color.GreenString("Use `ticloud update` to update to the latest version"))
+						fmt.Fprintln(h.IOStreams.Out, color.GreenString("Use `ticloud upgrade` to upgrade to the latest version"))
 					}
 				}
 			}