Update TheP8I package name in Externals CMakefile
TheP8I package is needed to use the Pythia8 hadronisation plugin for ThePEG. The name of the package has slightly changed in LCG_88b; this MR only updates the TheP8I CMakefile to match the correct name of TheP8I version that are available on cvmfs. While a new version of TheP8I is already requested, with this MR it can be validated if the current P8I version can be linked against the 'older' version of ThePEG.