Clean up E0120 long explanation

Author: GuillaumeGomez

src/librustc_error_codes/error_codes/E0120.md

@@ -1,5 +1,7 @@
-An attempt was made to implement Drop on a trait, which is not allowed: only
-structs and enums can implement Drop. An example causing this error:
+The Drop was implemented on a trait, which is not allowed: only structs and
+enums can implement Drop.
+
+Erroneous code example:
 
 ```compile_fail,E0120
 trait MyTrait {}
@@ -10,7 +12,7 @@ impl Drop for MyTrait {
 ```
 
 A workaround for this problem is to wrap the trait up in a struct, and implement
-Drop on that. An example is shown below:
+Drop on that:
 
 ```
 trait MyTrait {}
@@ -22,7 +24,7 @@ impl <T: MyTrait> Drop for MyWrapper<T> {
 
 ```
 
-Alternatively, wrapping trait objects requires something like the following:
+Alternatively, wrapping trait objects requires something:
 
 ```
 trait MyTrait {}