Remove match_length lexer user code block for Rust

CHANGELOG.md

@@ -10,7 +10,7 @@
   `match_text` for every target language. It was previously named `match` for
   C, C++, and D, and `match_` for Rust. Any lexer user code block which
   references the matched text must be updated to use the new name. The
-  `match_length` argument (C, C++, and Rust) is unchanged.
+  `match_length` argument (C and C++) is unchanged.
 - Tree generation mode now stores all tree nodes in a compact arena owned by
   the parser context (a flat node array plus a shared child-link array).
   This replaces the previous design of one heap allocation per node with