Auto merge of rust-lang#6451 - giraffate:update_contributing_md, r=ll…

…ogiq

Fix links in CONTRIBUTING.md

Links is broken.

<img width="1219" alt="スクリーンショット 2020-12-14 11 58 19" src="https://user-images.githubusercontent.com/17407489/102035564-fc152400-3e03-11eb-91a7-6c04e120d72f.png">

changelog: none

CONTRIBUTING.md

@@ -19,10 +19,10 @@ All contributors are expected to follow the [Rust Code of Conduct].
   - [Writing code](#writing-code)
   - [Getting code-completion for rustc internals to work](#getting-code-completion-for-rustc-internals-to-work)
   - [How Clippy works](#how-clippy-works)
-  - [Syncing changes between Clippy and [`rust-lang/rust`]](#syncing-changes-between-clippy-and-rust-langrust)
+  - [Syncing changes between Clippy and `rust-lang/rust`](#syncing-changes-between-clippy-and-rust-langrust)
     - [Patching git-subtree to work with big repos](#patching-git-subtree-to-work-with-big-repos)
-    - [Performing the sync from [`rust-lang/rust`] to Clippy](#performing-the-sync-from-rust-langrust-to-clippy)
-    - [Performing the sync from Clippy to [`rust-lang/rust`]](#performing-the-sync-from-clippy-to-rust-langrust)
+    - [Performing the sync from `rust-lang/rust` to Clippy](#performing-the-sync-from-rust-langrust-to-clippy)
+    - [Performing the sync from Clippy to `rust-lang/rust`](#performing-the-sync-from-clippy-to-rust-langrust)
     - [Defining remotes](#defining-remotes)
   - [Issue and PR triage](#issue-and-pr-triage)
   - [Bors and Homu](#bors-and-homu)