[doc] errors on Core:Enumerable#commonality

[chikamichi]

The block code in the doc for this method is erroneous.
http://facets.rubyforge.org/apidoc/api/core/classes/Enumerable.html#M000511

From an irb session:

ruby-1.9.1-p378 > [1, 2, 2, 3, 4, 4].commonality
 => {2=>[2, 2], 4=>[4, 4]} 
ruby-1.9.1-p378 > ["foo", "bar", "a"].commonality { |str| str.length }
 => {3=>["foo", "bar"]}

The collision method mentionned in the doc doesn't seem to be an existing alias of commonality (could be interesting to add it though!).

Thanks. Fixed on both counts.