Add documentation for metFileExt attribute on exec element in ExternMetExtractor

The CAS ExternMetExtractor allows the following attribute on its configuration file exec tag:

<cas:externextractor...>
 
  <exec metFileExt=".." ..>
 
  </exec>
..
</cas:externextractor>

This should be updated in the documentation in src/main/resources/examples/extern-config.xml