Add sample fix for E0749

Even though the description is clear but the solution may not be as straightforward.
Adding a suggested fix.

src/librustc_error_codes/error_codes/E0749.md

@@ -11,6 +11,7 @@ trait MyTrait {
 impl !MyTrait for u32 {
     type Foo = i32; // error!
 }
+// impl !MyTrait for u32 {} // fix
 # fn main() {}
 ```