Fix the repository URL in pubspec.yaml

The branch name `main` must be preceded by `tree`.

Author: sschuberth

pkgs/args/pubspec.yaml

@@ -3,7 +3,7 @@ version: 2.6.0
 description: >-
   Library for defining parsers for parsing raw command-line arguments into a set
   of options and values using GNU and POSIX style options.
-repository: https://github.com/dart-lang/core/main/pkgs/args
+repository: https://github.com/dart-lang/core/tree/main/pkgs/args
 
 topics:
   - cli