docs: Format CONTRIBUTING.md

* Correct auto-format command in CONTRIBUTING.md

* Format CONTRIBUTING.md

* Convert some inline code snippets to block code snippets.
* Delete trailing spaces.

CONTRIBUTING.md

@@ -105,28 +105,36 @@ Ruffle is built using the latest stable version of the Rust compiler. Nightly an
 
 The Rust code in Ruffle strives to be idiomatic. The Rust compiler should emit no warnings when building the project. Additionally, all code should be formatted using [`rustfmt`](https://github.com/rust-lang/rustfmt) and linted using [`clippy`](https://github.com/rust-lang/rust-clippy). You can install these tools using `rustup`:
 
-```
+```sh
 rustup component add rustfmt
 rustup component add clippy
 ```
 
 You can auto-format your changes with `rustfmt`:
 
-`cargo run fmt --all`
+```sh
+cargo fmt --all
+```
 
 and you can run the clippy lints:
 
-`cargo clippy --all --tests`
+```sh
+cargo clippy --all --tests
+```
 
 Specific warnings and clippy lints can be allowed when appropriate using attributes, such as:
 
-`#[allow(clippy::float_cmp)]`
+```rs
+#[allow(clippy::float_cmp)]
+```
 
 ## Commit Message Guidelines
 
 Here is a sample commit message:
 
-`web: Fix incorrect rendering of gradients (close #23)`
+```
+web: Fix incorrect rendering of gradients (close #23)
+```
 
  * If applicable, prefix the first line with a tag indicating the relevant area of changes:
    * `core:`