readme: official builds don't actually run on Mac OS X 10.5

Change-Id: I31c0ec00d08f1ddf0624f93d41d9ce5d9194039e
Reviewed-on: https://gerrit.libreoffice.org/11969
Reviewed-by: Norbert Thiebaud <nthiebaud@gmail.com>
Tested-by: Norbert Thiebaud <nthiebaud@gmail.com>