Add '-3' suffix to fftw libraries
Add '-3' suffix to fftw libraries

The fftw libraries now appear to have a '-3' suffix on them. Update
the documentation to reflect that.

Signed-off-by: Patrick Avery <743342299f279e7a8c3ff5eb40671fce3e95f13a@kitware.com>

OpenChemistry/tomviz-superbuild,cjh1/tomviz-superbuild,cjh1/tomviz-superbuild,OpenChemistry/tomviz-superbuild