message: core: fix link to `Metadata` in `Event`
  ## Motivation
  Current link in [here][1], which should point to the correct
  [`metadata`][2]() struct definition, instead points to [here][3].
  [1]:
  [2]:
  [3]:
  ## Solution
  Fix link. :)
diff: @@ -96,7 +96,7 @@ impl<'a> Event<'a> {
  /// Returns [metadata] describing this `Event`.
  ///
  - /// [metadata]: ../metadata/struct.Metadata.html
  + /// [metadata]: ../struct.Metadata.html
  pub fn metadata(&self) -> &'static Metadata<'static> {
  self.metadata
  }
  