Conflicting labels in tutorial.rdoc

There are two same labels `Defining Options`, [old one](https://github.com/ruby/optparse/pull/24/files#diff-847930dd3ac40523c1aedb290ee1acb7ef0d8b4611be6fef9cbdc70076d8c661R35) and [added at #24](https://github.com/ruby/optparse/pull/24/files#diff-847930dd3ac40523c1aedb290ee1acb7ef0d8b4611be6fef9cbdc70076d8c661R58).

            