Fix rst syntax (ros-controls#471)

Author: christophfroehlich

doc/migration.rst

@@ -8,5 +8,6 @@ This list summarizes important changes between Kilted Kaiju (previous) and Lyric
 RealtimeBox
 *******************************
 * ``RealtimePublisher`` is updated with a new ``try_publish`` API.
+
   * Update your code with a local message variable and call ``try_publish`` with that variable. (`#323 <https://github.com/ros-controls/realtime_tools/pull/323>`__).
   * ``msg_`` variable is inaccessible now (`#421 <https://github.com/ros-controls/realtime_tools/pull/421>`__).