Help: Modernize PackageConfigHelpers example

The example shown in the module documentation for CMakePackageConfigHelpers was showing its age, by:

1. Hardcoding directory prefixes (etc/, lib/, etc.) instead of using GNUInstallDirs
2. Handwaving set() commands (incorrectly!) as:
   set(VAR dir/ ... CACHE )
   which should, at the very least, be:
   set(VAR dir/ CACHE ... )
3. Installing CMake configuration files to lib/Foo/cmake/, when current practice favors ${CMAKE_INSTALL_LIBDIR}/cmake/Foo/

This updated example addresses all of those issues.