clean up E0393 explanation

Author: GuillaumeGomez

src/librustc_error_codes/error_codes/E0393.md

@@ -1,5 +1,6 @@
 A type parameter which references `Self` in its default value was not specified.
-Example of erroneous code:
+
+Erroneous code example:
 
 ```compile_fail,E0393
 trait A<T=Self> {}