Update details on how example files are filtered.
Update details on how example files are filtered.

- bringing instructions in line with how the x-api and configlet work
  (see exercism/configlet@27424c1e081d07544eb1009e3ecefbf5a7e62cba
  and exercism/xjava#50 for details).

amar47shah/x-api,lfex/exercism-api,jtigger/x-api,exercistas/x-api,jtigger/x-api,yurrriq/x-api,amar47shah/x-api,exercism/x-api,lfex/exercism-api,exercistas/x-api,exercism/x-api,yurrriq/x-api