internal: Fix rustdoc warnings

`cargo doc` generates a bunch of warnings on rust-analyzer. Fix all the
bare URL and empty code block warnings.

crates/parser/src/grammar.rs

@@ -13,7 +13,7 @@
 //! Code in this module also contains inline tests, which start with
 //! `// test name-of-the-test` comment and look like this:
 //!
-//! ```
+//! ```text
 //! // test function_with_zero_parameters
 //! // fn foo() {}
 //! ```