Fix typo in improper_ctypes suggestion

closes #52345

Author: Robin Kruppe

src/librustc_lint/types.rs

@@ -673,7 +673,7 @@ impl<'a, 'tcx> ImproperCTypesVisitor<'a, 'tcx> {
                         return FfiUnsafe {
                             ty: ty,
                             reason: "this function pointer has Rust-specific calling convention",
-                            help: Some("consider using an `fn \"extern\"(...) -> ...` \
+                            help: Some("consider using an `extern fn(...) -> ...` \
                                         function pointer instead"),
                         }
                     }

src/test/ui/lint-ctypes.stderr

@@ -126,15 +126,15 @@ error: `extern` block uses type `fn()` which is not FFI-safe: this function poin
 LL |     pub fn fn_type(p: RustFn); //~ ERROR function pointer has Rust-specific
    |                       ^^^^^^
    |
-   = help: consider using an `fn "extern"(...) -> ...` function pointer instead
+   = help: consider using an `extern fn(...) -> ...` function pointer instead
 
 error: `extern` block uses type `fn()` which is not FFI-safe: this function pointer has Rust-specific calling convention
   --> $DIR/lint-ctypes.rs:70:24
    |
 LL |     pub fn fn_type2(p: fn()); //~ ERROR function pointer has Rust-specific
    |                        ^^^^
    |
-   = help: consider using an `fn "extern"(...) -> ...` function pointer instead
+   = help: consider using an `extern fn(...) -> ...` function pointer instead
 
 error: `extern` block uses type `std::boxed::Box<u32>` which is not FFI-safe: this struct has unspecified layout
   --> $DIR/lint-ctypes.rs:71:28