Clean up E0077 long explanation

src/librustc_error_codes/error_codes/E0077.md

@@ -1,20 +1,23 @@
-When using the `#[simd]` attribute on a tuple struct, the elements in the tuple
-must be machine types so SIMD operations can be applied to them.
+A tuple struct's element isn't a machine type when using the `#[simd]`
+attribute.
 
-This will cause an error:
+Erroneous code example:
 
 ```compile_fail,E0077
 #![feature(repr_simd)]
 
 #[repr(simd)]
-struct Bad(String);
+struct Bad(String); // error!
 ```
 
-This will not:
+When using the `#[simd]` attribute on a tuple struct, the elements in the tuple
+must be machine types so SIMD operations can be applied to them.
+
+Fixed example:
 
 ```
 #![feature(repr_simd)]
 
 #[repr(simd)]
-struct Good(u32, u32, u32);
+struct Good(u32, u32, u32); // ok!
 ```