[docs] [issue25523] Correct "a" article to "an" article

Mark Dickinson added the comment: Yes, the Lib/test/decimaltestdata files do come from an external source. I don't see the harm in including them in the changes, but those changes will be overwritten when the files are next updated. ---------- _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue25523> _______________________________________