Fix a simple typo in the description. (ros2#71)

Signed-off-by: Chris Lalancette <[email protected]>

Author: clalancette

actionlib_msgs/package.xml

@@ -3,7 +3,7 @@
 <package format="3">
   <name>actionlib_msgs</name>
   <version>0.7.0</version>
-  <description>A package containing some message definitions used in the implementation or actions.</description>
+  <description>A package containing some message definitions used in the implementation of actions.</description>
   <maintainer email="[email protected]">William Woodall</maintainer>
   <license>Apache License 2.0</license>