Fix a code block containing ```rust

This wasn't displayed correctly.
Changing `` ```rust `` to `` ```rust`` made it a code block again,
but the leading space was rendered as-well.
(This may be an issue in mdbook, but I'm not sure.)

This commit changes it to use HTML-syntax in order to get the desired result.

src/doc/rustc-dev-guide/src/rustdoc-internals.md

@@ -72,7 +72,7 @@ Here is the list of passes as of <!-- date: 2021-02 --> February 2021:
   flag.
 
 - `check-code-block-syntax` validates syntax inside Rust code blocks
-  (`` ```rust ``)
+  (<code>```rust</code>)
 
 - `check-invalid-html-tags` detects invalid HTML (like an unclosed `<span>`)
   in doc comments.