Very slight improvement to documentation.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@3389 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/set.rb

@@ -100,7 +100,7 @@ class Set
     self
   end
 
-  # Converts the set to an array. (the order is uncertain)
+  # Converts the set to an array.  The order of elements is uncertain.
   def to_a
     @hash.keys
   end