Link to real wrapper (macs) that tests output composite files...

Author: jmchilton

docs/writing_how_do_i.rst

@@ -201,6 +201,7 @@ specify tests for the individual output files using the ``extra_files`` element
 demonstrated by the following tool.
 
 - `composite_output.xml <https://github.com/galaxyproject/galaxy/blob/dev/test/functional/tools/composite_output.xml>`__
+- `macs_wrapper.xml <https://github.com/galaxyproject/tools-devteam/blob/master/tools/macs/macs_wrapper.xml>`__
 
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 \.\.\. test index (\.loc) data?