Point2d/3d circumcenterHelp function name typo fix

[dearlordylord]

Pull request checklist:

• Is the pull request from a dedicated branch in your fork instead of
  master? (Preferred, but not mandatory.)

no

• Have you added yourself to the AUTHORS file? Not mandatory if you'd prefer
  to stay anonymous, but don't be shy!

no; it's not an impactful change

• Is "Allow edits from maintainers" enabled? (There should be a checkbox for
  this when you open the pull request.)

yes

• Is the code formatted using the same version of elm-format as the rest of
  elm-geometry? (If running elm-format doesn't change any existing code,
  then the answer is probably 'yes'.)

yes

• Is code (mostly) wrapped to 80 columns? (It's OK if type annotations and
  occasional things like string literals are longer.)

yes