Fix error code tests

Author: rylev

compiler/rustc_error_codes/src/error_codes/E0782.md

@@ -2,7 +2,7 @@ Trait objects must include the `dyn` keyword.
 
 Erroneous code example:
 
-```edition2021,compile_fail,E782
+```edition2021,compile_fail,E0782
 trait Foo {}
 fn test(arg: Box<Foo>) {} // error!
 ```

compiler/rustc_error_codes/src/error_codes/E0783.md

@@ -2,7 +2,7 @@ The range pattern `...` is no longer allowed.
 
 Erroneous code example:
 
-```edition2021,compile_fail,E783
+```edition2021,compile_fail,E0783
 match 2u8 {
     0...9 => println!("Got a number less than 10"), // error!
     _ => println!("Got a number 10 or more"),