Fixed monthly archives to respect MONTH_ARCHIVE_SAVE_AS setting

The foundation-default-colours theme ignores the format in MONTH_ARCHIVE_SAVE_AS, using the example format from the docs instead.
This change uses the MONTH_ARCHIVE_SAVE_AS format string directly.

(This is a repost of [209](https://github.com/getpelican/pelican-themes/pull/209). I'm new to git and created that one from my master branch.)