Fix README

Fix methods called on AdminAuthorizer
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The testing section of the README refers to the method bulk_updatable_by(). After trying to add tests it seems that the methods defined on the class are still just updatable_by() [NOTE: lack of bulk_ prefix] like the instance methods. Feel free to ignore the PR if I'm mistaken.

PS. Thanks for a cool gem. It's really helping me keep code cleaner and more readable.