Refer to master branch only and drop reference to trunk branch name

Author: Gary Gregory

CONTRIBUTING.md

@@ -61,7 +61,7 @@ Making Changes
 --------------
 
 + Create a _topic branch_ for your isolated work.
-  * Usually you should base your branch on the `master` or `trunk` branch.
+  * Usually you should base your branch on the `master` branch.
   * A good topic branch name can be the JIRA bug id plus a keyword, e.g. `IO-123-InputStream`.
   * If you have submitted multiple JIRA issues, try to maintain separate branches and pull requests.
 + Make commits of logical units.