Improve collapsible_match error message syntax

clippy_lints/src/matches/collapsible_match.rs

@@ -96,7 +96,7 @@ fn check_arm<'tcx>(
         // collapsing patterns need an explicit field name in struct pattern matching
         // ex: Struct {x: Some(1)}
         let replace_msg = if is_innermost_parent_pat_struct {
-            format!(", prefixed by {}:", snippet(cx, binding_span, "their field name"))
+            format!(", prefixed by `{}`:", snippet(cx, binding_span, "their field name"))
         } else {
             String::new()
         };

tests/ui/collapsible_match.stderr

@@ -223,7 +223,7 @@ help: the outer pattern can be modified to include the inner pattern
 LL |     if let Issue9647::A { a, .. } = x {
    |                           ^ replace this binding
 LL |         if let Some(u) = a {
-   |                ^^^^^^^ with this pattern, prefixed by a:
+   |                ^^^^^^^ with this pattern, prefixed by `a`:
 
 error: this `if let` can be collapsed into the outer `if let`
   --> tests/ui/collapsible_match.rs:292:9