[front_end] Standardize on one way to tell user to update messages.

These two commands are equivalent:
    dart pkg/front_end/tool/cfe.dart generate-messages
    dart run pkg/front_end/tool/generate_messages.dart

To reduce confusion, always use the former form when telling the user
what to do to update generated messages files.

Change-Id: I6a6a69644021a68c07b81ecce86b37141a4dbc2c
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/447560
Reviewed-by: Johnni Winther <[email protected]>
Commit-Queue: Johnni Winther <[email protected]>
Auto-Submit: Paul Berry <[email protected]>

Author: stereotype441

pkg/front_end/messages.yaml

@@ -4,7 +4,7 @@
 
 # Run
 #
-#     dart run pkg/front_end/tool/generate_messages.dart
+#     dart pkg/front_end/tool/cfe.dart generate-messages
 #
 # to regenerate messages after having edited this file.
 #