accept review suggestion

Co-authored-by: nils <48135649+Nilstrieb@users.noreply.github.com>

Author: tshepang

compiler/rustc_middle/src/mir/visit.rs

@@ -3,7 +3,7 @@
 //! ## Overview
 //!
 //! There are two visitors, one for immutable and one for mutable references,
-//! but both are generated by `make_mir_visitor` macro.
+//! but both are generated by the `make_mir_visitor` macro.
 //! The code is written according to the following conventions:
 //!
 //! - introduce a `visit_foo` and a `super_foo` method for every MIR type