Update CONTRIBUTING.md

Author: dmronga

CONTRIBUTING.md

@@ -22,7 +22,7 @@ The preferred way to contribute to wbc_msgs is to fork the main repository on Gi
 
         $ git checkout -b my-feature
 
-    and start making changes. Never work in the ``master`` branch!
+    and start making changes. Never work in the ``main`` branch!
 
 5. Work on this copy, on your computer, using Git to do the version
    control. When you're done editing, do::