update rust repo default branch name

Author: marcoieni

draft/DRAFT_TEMPLATE

@@ -10,7 +10,7 @@ Want something mentioned? Tag us at
 [@thisweekinrust.bsky.social](https://bsky.app/profile/thisweekinrust.bsky.social) on Bluesky or
 [@ThisWeekinRust](https://mastodon.social/@thisweekinrust) on mastodon.social, or
 [send us a pull request](https://github.com/rust-lang/this-week-in-rust).
-Want to get involved? [We love contributions](https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md).
+Want to get involved? [We love contributions](https://github.com/rust-lang/rust/blob/main/CONTRIBUTING.md).
 
 *This Week in Rust* is openly developed [on GitHub](https://github.com/rust-lang/this-week-in-rust) and archives can be viewed at [this-week-in-rust.org](https://this-week-in-rust.org/).
 If you find any errors in this week's issue, [please submit a PR](https://github.com/rust-lang/this-week-in-rust/pulls).