fix bug in Levenshtein distance calculation

Bug was discovered and discussed in #18395.

guides/rails_guides/levenshtein.rb

@@ -7,11 +7,9 @@ def self.distance str1, str2
       t = str2
       n = s.length
       m = t.length
-      max = n/2
 
       return m if (0 == n)
       return n if (0 == m)
-      return n if (n - m).abs > max
 
       d = (0..m).to_a
       x = nil