code: func (s *AppsService) GetInstallation(ctx context.Context, id int64) (*Installation, *Response, error) {
  	return s.getInstallation(ctx, fmt.Sprintf("app/installations/%v", id))
  }
docs: // GetInstallation returns the specified installation.
  //
  // GitHub API docs: https://developer.github.com/v3/apps/#get-a-single-installation