checker: fix a string error test

vlib/v/checker/tests/string_index_assign_error.out

@@ -1,4 +1,5 @@
-vlib/v/checker/tests/string_index_assign_error.v:3:6: error: cannot assign to s[i] (strings are immutable)
+vlib/v/checker/tests/string_index_assign_error.v:3:6: error: cannot assign to s[i] since V strings are immutable
+(note, that variables may be mutable but string values are always immutable, like in Go and Java)
     1 | fn main() {
     2 |     mut a := 'V is great!!'
     3 |     a[0] = `R`