Clean up E0326 explanation

Author: GuillaumeGomez

src/librustc_error_codes/error_codes/E0326.md

@@ -1,7 +1,6 @@
-The types of any associated constants in a trait implementation must match the
-types in the trait definition. This error indicates that there was a mismatch.
+An implementation of a trait doesn't match the type contraint.
 
-Here's an example of this error:
+Erroneous code example:
 
 ```compile_fail,E0326
 trait Foo {
@@ -14,3 +13,6 @@ impl Foo for Bar {
     const BAR: u32 = 5; // error, expected bool, found u32
 }
 ```
+
+The types of any associated constants in a trait implementation must match the
+types in the trait definition.