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

Martin Panter added the comment: Thanks Serhiy. My pass over the files missed most cases where there was a line break involved, and certain punctuation and markup, which you picked up on. I skipped the /Lib/msilib/schema.py file in my patch because it was part of the data rather than a comment, and I wasn’t sure what it is for. It seems to be a generated file from some Windows source. But I see other people have made similar corrections, so it might be okay. I left a couple of comments. Apart from those, the rest look good to me. ---------- _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue25523> _______________________________________