The new issue template should clarify dart-lang/lints vs. dart-lang/linter

[jamesderlin]

I wanted to file a request for a new lint, so I typed github.com/dart-lang/lint in my browser address bar and let it autocomplete. I wasn't paying close attention and forgot that there are separate dart-lang/lints and dart-lang/linter repositories. (The only reason I noticed was that after I submitted, I realized that the issue number was suspiciously low.)

The new issue template for dart-lang/lints probably should direct people to dart-lang/linter for new lint requests and emphasize that dart-lang/lints is for the officially recommended set of lints.