Clean up E0512 explanation

Author: GuillaumeGomez

src/librustc_error_codes/error_codes/E0512.md

@@ -1,5 +1,6 @@
-Transmute with two differently sized types was attempted. Erroneous code
-example:
+Transmute with two differently sized types was attempted.
+
+Erroneous code example:
 
 ```compile_fail,E0512
 fn takes_u8(_: u8) {}