chore(mason_api): update documentation in pubspec.yaml

Author: felangel

packages/mason_api/pubspec.yaml

@@ -4,7 +4,7 @@ version: 0.1.0-dev.12
 homepage: https://github.com/felangel/mason
 repository: https://github.com/felangel/mason
 issue_tracker: https://github.com/felangel/mason/issues
-documentation: https://github.com/felangel/mason/tree/master/packages/mason_cli#readme
+documentation: https://github.com/felangel/mason/tree/master/packages/mason_api#readme
 topics: [mason, template, generator]
 funding: [https://github.com/sponsors/felangel]