(maint) doc fix - there is no Facter::Util::Execution

Yard docs had a reference to `Facter::Util::Execution` (does not exist) instead of `Facter::Core::Execution`
Merge pull request #817 from jhoblitt/maint/doc_typo_facter_util_execution

(maint) doc fix - there is no Facter::Util::Execution