clean up error codes explanation

Author: GuillaumeGomez

src/librustc_error_codes/error_codes/E0220.md

@@ -1,4 +1,5 @@
-You used an associated type which isn't defined in the trait.
+A used associated type wasn't defined in the trait.
+
 Erroneous code example:
 
 ```compile_fail,E0220

src/librustc_error_codes/error_codes/E0221.md

@@ -1,5 +1,6 @@
 An attempt was made to retrieve an associated type, but the type was ambiguous.
-For example:
+
+Erroneous code example:
 
 ```compile_fail,E0221
 trait T1 {}

src/librustc_error_codes/error_codes/E0222.md

@@ -1,5 +1,6 @@
 An attempt was made to constrain an associated type.
-For example:
+
+Erroneous code example:
 
 ```compile_fail,E0222
 pub trait Vehicle {