Github short form plugin source doesn't handle branches with slash

Docs (https://buildkite.com/docs/agent/v3/plugins#plugin-sources) describe a short form of the github plugin source (e.g. foo/bar#baz = https://github.com/foo/bar-buildkite-plugin.git#baz), however this doesn't work with a branch containing a slash:

```
- label: "Deploy"
plugins:
healthefrog/ecs-deploy#feature/fix_task_definition:
```

produces a url of `https:/healthefrog/ecs-deploy#feature/fix_task_definition`

Previously raised on docs issue tracker: https://github.com/buildkite/docs/issues/265