Merge pull request ros-simulation#525 from lucasw/patch-1

Fix spelling of highly

CONTRIBUTING.md

@@ -4,7 +4,7 @@ We welcome contributions to this repo and encourage you to fork the project, tha
 
 ## Implementations of simulations
 
-It is higly recommended to place the code that performs simulation inside
+It is highly recommended to place the code that performs simulation inside
 [upstream gazebo code](https://bitbucket.org/osrf/gazebo/). The Gazebo project
 provides documentation about [how to create and code
 plugins](http://gazebosim.org/tutorials?cat=write_plugin) Ideally