Increase font-size on callout arrow

I can only imagine this fixes the issue because the govuk_template
is using a slightly different base font size to static.

Regardless, this does improve the display of the "Users at each stage"
module in Chrome, Firefox, Safari on OS X.
Merge pull request #149 from alphagov/63818330-a-whole-lott-of-bugfix

Increase font-size on callout arrow