Link to this repo's guide in the PR template (#4033)

The plugins repo contributor's guide mostly defers to the main one, which it links to, but includes repository-specific information.

Also simplifies the section on style guides, while making it more comprehensive, by linking to the style section of the contributor guide.
diff --git a/.github/PULL_REQUEST_TEMPLATE.md b/.github/PULL_REQUEST_TEMPLATE.md
index d470ac1..3972cd2 100644
--- a/.github/PULL_REQUEST_TEMPLATE.md
+++ b/.github/PULL_REQUEST_TEMPLATE.md
@@ -8,7 +8,7 @@
 
 - [ ] I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
 - [ ] I read the [Tree Hygiene] wiki page, which explains my responsibilities.
-- [ ] I read and followed the [Flutter Style Guide] and the [C++, Objective-C, Java style guides]. (Note that unlike the flutter/flutter repo, the flutter/plugins repo does use `dart format`. See [plugin_tool format])
+- [ ] I read and followed the [relevant style guides] and ran [the auto-formatter]. (Note that unlike the flutter/flutter repo, the flutter/plugins repo does use `dart format`.)
 - [ ] I signed the [CLA].
 - [ ] The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. `[shared_preferences]`
 - [ ] I listed at least one issue that this PR fixes in the description above.
@@ -21,13 +21,12 @@
 If you need help, consider asking for advice on the #hackers-new channel on [Discord].
 
 <!-- Links -->
-[Contributor Guide]: https://github.com/flutter/flutter/wiki/Tree-hygiene#overview
+[Contributor Guide]: https://github.com/flutter/plugins/blob/master/CONTRIBUTING.md
 [Tree Hygiene]: https://github.com/flutter/flutter/wiki/Tree-hygiene
-[Flutter Style Guide]: https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo
-[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/master/CONTRIBUTING.md#style
+[relevant style guides]: https://github.com/flutter/plugins/blob/master/CONTRIBUTING.md#style
 [CLA]: https://cla.developers.google.com/
 [flutter/tests]: https://github.com/flutter/tests
 [breaking change policy]: https://github.com/flutter/flutter/wiki/Tree-hygiene#handling-breaking-changes
 [Discord]: https://github.com/flutter/flutter/wiki/Chat
 [pub versioning philosophy]: https://dart.dev/tools/pub/versioning
-[plugin_tool format]: https://github.com/flutter/plugins/blob/master/script/tool/README.md#format-code
+[the auto-formatter]: https://github.com/flutter/plugins/blob/master/script/tool/README.md#format-code