checker: fix mut int test

vlib/v/checker/tests/mut_int.out

@@ -1,6 +1,6 @@
 vlib/v/checker/tests/mut_int.v:1:12: error: mutable arguments are only allowed for arrays, maps, and structs
 return values instead: `fn foo(n mut int) {` => `fn foo(n int) int {`
-    1 | fn foo(mut x int)  {
+    1 | fn foo(mut x int) {
       |            ^
     2 | }
     3 |