Change "appendixes" to "appendices" in intro.

[tombusby]

As per https://en.wiktionary.org/wiki/appendix, the OED states that:

Appendix typically has the plural appendixes in the anatomical sense, and appendices when referring to a part of a book or document.

To me, the use of "appendixes" in the text reads as incorrect. However, Mirriam-Webster doesn't seem to make a strong recommendation between the two, so it's possible that this sense of incorrectness stems from the fact that I'm a native British-English speaker.

I like to try to contribute small spelling and grammar fixes when I'm reading docs. However, this correction is perhaps a matter of personal style to some degree. I'd argue that fewer people will have the nagging sense of "that's a mistake" when reading "appendices" than "appendixes" though, so I'll put this change to the maintainers for consideration.

[stappersg]

https://en.wiktionary.org/wiki/appendix also say that both are fine.

@tombusby your sharp eye on spelling and grammar is appreciated. ( Don't ask by who many ;-)
Please don't let slow response on pull request hinder you.

[tombusby]

@stappersg I think it's still optimal to make the change. American English speakers will not consider either to be incorrect, but most British English speakers will find that one sticks out to them as a mistake. Going with the one that no-one considers a mistake seems like the best bet. But it's not a major issue, it's just something that I found jarring when I was reading it. I'm happy with it either being merged or not.

[carols10cents]

Sure, seems fine. Thanks!