Make trailing colon on class manipulation clear

I got stuck on class manipulation because I thought that the trailing colon on the documentation was just a typo and therefore didn't include it in my code. I only found out after diving into the code and realizing that `axisPattern = new RegExp('^(data-)?(in|intent)-([a-zA-Z0-9][_a-zA-Z0-9]*):$');` indeed has a `:` near the end. So I think it should be more clear on the doc.