* lib/set.rb: [DOC] Fix grammatical error by @bouk [fix GH-524]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44812 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/set.rb

@@ -541,7 +541,7 @@ class Set
 end
 
 #
-# SortedSet implements a Set that guarantees that it's element are
+# SortedSet implements a Set that guarantees that its elements are
 # yielded in sorted order (according to the return values of their
 # #<=> methods) when iterating over them.
 #