Update Chapel help message for --log

The old version said the definition of the letters to use lived in log.h.  That
is no longer true, they live in runpasses.cpp.  Update the help message to point
to the right file.
Merge pull request #742 from lydia-duncan/updateHelpMessage

Update Chapel help message for --log

The old version said the definition of the letters to use lived in log.h. That
is no longer true as of June, they live in runpasses.cpp. Update the help
message to point to the right file.