Clear up inconsistency in authorize_resource examples

Forward
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While reading through the source code and its examples, I noticed what I believe is an inconsistency in the example code and example explanation for `authorize_resource`.

I created two comments on the related lines in the commit where this was introduced. The comments are asking whether my hunch is correct.

https://github.com/ryanb/cancan/commit/25a1c553bfefafd3002bddfc67152a27bd0eb3d7#L2R115
https://github.com/ryanb/cancan/commit/25a1c553bfefafd3002bddfc67152a27bd0eb3d7#L2R118

This pull request is created in case my hunch proves correct, and the documentation needs to be updated to reflect that.

Change Description
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Changed the explanation of the code in the example to have the same references and order as the related example code.

Previously, the example code referenced a `book`, whereas the explanation of that example code mentioned a `show`. The example and the explanation did not match.