clean up E0197 explanation

Author: GuillaumeGomez

src/librustc_error_codes/error_codes/E0197.md

@@ -1,13 +1,20 @@
+An inherent implementation was marked unsafe.
+
+Erroneous code example:
+
+```compile_fail,E0197
+struct Foo;
+
+unsafe impl Foo { } // error!
+```
+
 Inherent implementations (one that do not implement a trait but provide
 methods associated with a type) are always safe because they are not
 implementing an unsafe trait. Removing the `unsafe` keyword from the inherent
 implementation will resolve this error.
 
-```compile_fail,E0197
+```
 struct Foo;
 
-// this will cause this error
-unsafe impl Foo { }
-// converting it to this will fix it
-impl Foo { }
+impl Foo { } // ok!
 ```