Polonius team: fix path specified under "relevant repositories" section for the borrow check crate in the main Rust repo

[bindsdev]

On the Polonius Working Group Web Page, under the Relevant Repositories bullet in the How can I get involved? section, it specifies the rust-lang/rust repository, but specifically the path src/librustc_mir/borrow_check. In the current state of the rust-lang/rust repository, that path no longer exists. It was moved to compiler/rustc_borrowck as of tag 1.57.0 (September 6th, 2021 for a specific date) in the repository and still stands here to the present day. This change should be made so any newcomers that aren't particularly familiar with the rust-lang/rust repository are not misled by the now error in the writing.