Clean up E0520 explanation

Author: GuillaumeGomez

src/librustc_error_codes/error_codes/E0520.md

@@ -1,5 +1,7 @@
 A non-default implementation was already made on this type so it cannot be
-specialized further. Erroneous code example:
+specialized further.
+
+Erroneous code example:
 
 ```compile_fail,E0520
 #![feature(specialization)]