Fix typo in test suite.

I'm sure this typo was a discreet way of avoiding the discussion about
whether sanitisation should be spelled with an 's' or a 'z'. I'm going
to make an executive decision about this: It's 's'.

This has been my standard for quite some time*.

*)Exhibit A: the timetress gem. Commit made in December 2012:
https://github.com/kytrinyx/timetress/blob/df3b8f94e9b55be5d999fd10d3521250b6679e9f/lib/timetress/norway/holiday.rb#L14-L17