message: fix scopes docs
diff: -/// Create a **Hashset** from a list of &str(which will be converted to
  -/// String internally), be used to create scope for
  +/// Create a [`HashSet`](std::collections::HashSet) from a list of `&str` (which
  +/// will be converted to String internally), to easily create scopes for
  /// [`Token`](crate::oauth2::Token).
  ///
  /// Example:
  