swap function order for better read flow

When having the order

```
foo.bar(); // we can now use this method since i32 implements the Foo trait

[...]

impl Foo for i32
```

the `// we can now use this method` comment is less clear to me.

Author: RafaelKr

compiler/rustc_error_codes/src/error_codes/E0277.md

@@ -29,16 +29,16 @@ trait Foo {
     fn bar(&self);
 }
 
-fn some_func<T: Foo>(foo: T) {
-    foo.bar(); // we can now use this method since i32 implements the
-               // Foo trait
-}
-
 // we implement the trait on the i32 type
 impl Foo for i32 {
     fn bar(&self) {}
 }
 
+fn some_func<T: Foo>(foo: T) {
+    foo.bar(); // we can now use this method since i32 implements the
+               // Foo trait
+}
+
 fn main() {
     some_func(5i32); // ok!
 }