Incorrect example for lifecycle_rules

[bmccutchon]

The "note" on Bucket.lifecycle_rules acts as if lifecycle_rules returns a list, but it actually returns a generator.

[andrewsg]

Fixing the docstring on this now; I'd like to actually make it consistent with the setter (returning a list) or use some other consistent interface, but can't without breaking backwards compatibility, so this is now on the list for 2.0.