message: Update tracing.rst
  Correct the typo zipkin link
diff: @@ -23,7 +23,7 @@ driver
  *(optional, object)* Provides the driver that handles trace and span creation.
  Currently `LightStep <http://lightstep.com/>`_ and `Zipkin
  -<http://zipkin.io>` drivers are supported.
  +<http://zipkin.io>`_ drivers are supported.
  LightStep driver
  ----------------
  