Fix documenation that gives a scons warning

[rearmstr]

When running scons, the following message appears:

  parameter 'dataRef'
/sps/lsst/HSC/Manal/dia_pipe/python/lsst/dia/pipe/forcedPhotCoaddDiaSrc.py:106: warning: argument 'psfCache' of command @param is not found in the argument list of lsst.dia.pipe.forcedPhotCoaddDiaSrc.ForcedPhotCoaddDiaSrcTask::runDataRef(self, dataRef)

This should be fixed so scons doesn't give any warnings