rust-matchers: fixing cargo doc

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

--- a/rust/hg-core/src/matchers.rs	Wed Dec 25 15:17:55 2019 +0100
+++ b/rust/hg-core/src/matchers.rs	Wed Dec 25 19:03:07 2019 +0100
@@ -43,6 +43,7 @@
     /// visit_children_set is capable of recognizing this; some implementations
     /// are not).
     ///
+    /// ```text
     /// ```ignore
     /// '' -> {'foo', 'qux'}
     /// 'baz' -> set()