Fix docs highlight in `dict_iter_missing_items.rs` #37873

Job	Run time
Determine changes	12s
cargo fmt	19s
cargo build (release)	0s
mkdocs	2m 27s
pre-commit	3m 3s
python package	1m 22s
cargo fuzz build	5m 14s
benchmarks	5m 52s
cargo clippy	1m 59s
cargo build (msrv)	4m 11s
cargo shear	22s
cargo test (linux)	1m 55s
cargo test (linux, release)	3m 47s
cargo test (wasm)	1m 38s
cargo test (windows)	4m 10s
formatter instabilities and black similarity	0s
test scripts	57s
test ruff-lsp	38s
ecosystem	2m 36s
fuzz parser	0s
	40m 42s

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