docs: add --locked to cargo install commands (tokio-rs#6479)

CONTRIBUTING.md

@@ -172,7 +172,7 @@ as docs.rs itself does.
 [`cargo docs-rs`]: https://github.com/dtolnay/cargo-docs-rs
 
 ```
-cargo install cargo-docs-rs
+cargo install --locked cargo-docs-rs
 cargo +nightly docs-rs [--open]
 ```
 
@@ -261,7 +261,7 @@ directory `fuzz`. It is a good idea to run fuzz tests after each change.
 To get started with fuzz testing you'll need to install
 [cargo-fuzz](https://github.com/rust-fuzz/cargo-fuzz).
 
-`cargo install cargo-fuzz`
+`cargo install --locked cargo-fuzz`
 
 To list the available fuzzing harnesses you can run;