Clean up E0118 error code long explanation

Author: GuillaumeGomez

src/librustc_error_codes/error_codes/E0118.md

@@ -1,5 +1,7 @@
-You're trying to write an inherent implementation for something which isn't a
-struct nor an enum. Erroneous code example:
+An inherent implementation was defined for something which isn't a struct nor
+an enum.
+
+Erroneous code example:
 
 ```compile_fail,E0118
 impl (u8, u8) { // error: no base type found for inherent implementation