Fix invalid URL suggestion for gradle incompatability

[Dispersia]

The current URL has 2 /to, but it should only be 1. However, this url redirects to https://docs.flutter.dev/release/breaking-changes/android-java-gradle-migration-guide, so I can change it to that if it's preferred just to go to the final destination - just don't know if it's a redirect for a reason

Pre-launch Checklist

• [x] I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• [x] I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• [x] I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement].
• [x] I signed the [CLA].
• [ ] I listed at least one issue that this PR fixes in the description above.
• [x] I updated/added relevant documentation (doc comments with ///).
• [x] I added new tests to check the change I am making, or this PR is [test-exempt].
• [x] I followed the [breaking change policy] and added [Data Driven Fixes] where supported.
• [x] All existing and new tests are passing.

i didn't create an issue as it's a doc change, but if that's a requirement I can do that