Fixed around callback with lambda example

[aaronrenner]

The existing example of using yield in an around callback lambda throws
a "no block given (yield)" error. After some troubleshooting, I came up
with this example that works in a lambda.

[steveklabnik]

Neat! However, this should really be targeted at master, not 3-2-stable. Or at least, both. Also, feel free to push changes to lifo/docrails in the future as well.

[josevalim]

Thanks @aaronrenner . Since this is a documentation change, please push it straight to http://github.com/lifo/docrails! And please push it to master, after all, Rails 4 is coming sooner than later. :)