Fix names collector

The new `@root` syntax needs to be accounted here.

Author: ehuss

mdbook-spec/src/grammar.rs

@@ -136,7 +136,7 @@ impl Expression {
 static GRAMMAR_RE: LazyLock<Regex> =
     LazyLock::new(|| Regex::new(r"(?ms)^```grammar,([^\n]+)\n(.*?)^```").unwrap());
 static NAMES_RE: LazyLock<Regex> =
-    LazyLock::new(|| Regex::new(r"(?m)^([A-Za-z0-9_]+)(?: \([^)]+\))? ->").unwrap());
+    LazyLock::new(|| Regex::new(r"(?m)^(?:@root )?([A-Za-z0-9_]+)(?: \([^)]+\))? ->").unwrap());
 
 /// Loads the [`Grammar`] from the book.
 pub fn load_grammar(book: &Book, diag: &mut Diagnostics) -> Grammar {