docs: style guide for builders still references proc()

[Ryman]

http://doc.rust-lang.org/nightly/style/ownership/builders.html#consuming-builders

[steveklabnik]

Yes, in general the style guide hasn't been touched since just before 1.0, I'm sure there are a few of these. It's why it's not linked anywhere.

[frewsxcv]

The style guide was moved out of rust-lang/rust (this repository) some time ago to rust-lang/rust-guidelines. Since then, despite the prop() call still existing in the guidelines, rust-lang/rust-guidelines has been become discontinued, so I'm going to close this issue as there's not any action to take for this.