rust-matchers: fixing cargo doc

Block quotes marked as `ignore` are still parsed as Rust
source examples.

Differential Revision: https://phab.mercurial-scm.org/D7783

rust/hg-core/src/matchers.rs

@@ -43,6 +43,7 b' pub trait Matcher {'
     /// visit_children_set is capable of recognizing this; some implementations
     /// are not).
     ///
+    /// ```text
     /// ```ignore
     /// '' -> {'foo', 'qux'}
     /// 'baz' -> set()