[homebrew] Modify the version string when updating the version
[homebrew] Modify the version string when updating the version

This is a follow-up to a96b4f6 which changed the script but introduced
the bug that it no longer updated the version string in the formula.

Fixes https://github.com/dart-lang/sdk/issues/40990.

Change-Id: I272d21d2c4a44161f697ba52da6bb74b1cc7c5f9
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/142147
Reviewed-by: William Hesse <a821cddceae7dc400f272e3cb1a72f400f9fed6d@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk