Fix wrong usage example in a comment
Fix wrong usage example in a comment

According to the test: https://github.com/libgit2/rugged/blob/development/test/walker_test.rb#L53
Acidburn0zzz/rugged,saraid/rugged,tpickett66/rugged,libgit2/rugged,Acidburn0zzz/rugged,libgit2/rugged,tpickett66/rugged,saraid/rugged,Acidburn0zzz/rugged,tpickett66/rugged,saraid/rugged,libgit2/rugged