Cleanup E0390 explanation

Author: GuillaumeGomez

src/librustc_error_codes/error_codes/E0390.md

@@ -1,4 +1,6 @@
-You tried to implement methods for a primitive type. Erroneous code example:
+A method was implemented on a primitive type.
+
+Erroneous code example:
 
 ```compile_fail,E0390
 struct Foo {