Fix tags for Color#euclidean_distance_rgba

When I implemented this method, I accidentally left an older variable
name I was using in the YARD tags. Although these colors are
interchangeable due to the math involved (i.e. the before and after
aspect doesn't actually matter), the documentation should at least
reflect the actual code.
Merge pull request #64 from lencioni/euclidean-docs

Fix tags for Color#euclidean_distance_rgba