parser: update check for non mut types in fn

vlib/v/checker/tests/function_arg_mutable_err.out

@@ -1,5 +1,5 @@
 vlib/v/checker/tests/function_arg_mutable_err.v:1:18: error: mutable arguments are only allowed for arrays, maps, and structs
-return values instead: `fn foo(n mut int) {` => `fn foo(n int) int {`
+return values instead: `fn foo(mut n int) {` => `fn foo(n int) int {`
     1 | fn mod_ptr(mut a int) {
       |                  ~~~
     2 |     a = 77

vlib/v/checker/tests/mut_int.out

@@ -1,5 +1,5 @@
 vlib/v/checker/tests/mut_int.v:1:14: error: mutable arguments are only allowed for arrays, maps, and structs
-return values instead: `fn foo(n mut int) {` => `fn foo(n int) int {`
+return values instead: `fn foo(mut n int) {` => `fn foo(n int) int {`
     1 | fn foo(mut x int) {
       |              ~~~
     2 | }

vlib/v/parser/fn.v

@@ -458,7 +458,7 @@ fn (mut p Parser) check_fn_mutable_arguments(typ table.Type, pos token.Position)
 	sym := p.table.get_type_symbol(typ)
 	if sym.kind !in [.array, .struct_, .map, .placeholder] && !typ.is_ptr() {
 		p.error_with_pos('mutable arguments are only allowed for arrays, maps, and structs\n' +
-			'return values instead: `fn foo(n mut int) {` => `fn foo(n int) int {`', pos)
+			'return values instead: `fn foo(mut n $sym.name) {` => `fn foo(n $sym.name) $sym.name {`', pos)
 	}
 }