Fix name of function in exercise

Noticed in #458. The function name was changed from `normalize` to `center`, but the exercise still referred to `normalize`.
The filename `aux` was reserved in the days of DOS, and that proscription has been retained to the present day.  This PR therefore renames the recently-created `aux` directory to `misc` so that people can pull this repository on Windows.
Merge pull request #464 from gvwilson/renaming-aux-to-misc

Changing aux to misc to avoid problems on Windows
Merge branch 'master' into adding-kramdown-alds