clean up e0200 explanation

Author: GuillaumeGomez

src/librustc_error_codes/error_codes/E0200.md

@@ -1,14 +1,23 @@
+An unsafe trait was implemented without an unsafe implementation.
+
+Erroneous code example:
+
+```compile_fail,E0200
+struct Foo;
+
+unsafe trait Bar { }
+
+impl Bar for Foo { } // error!
+```
+
 Unsafe traits must have unsafe implementations. This error occurs when an
 implementation for an unsafe trait isn't marked as unsafe. This may be resolved
 by marking the unsafe implementation as unsafe.
 
-```compile_fail,E0200
+```
 struct Foo;
 
 unsafe trait Bar { }
 
-// this won't compile because Bar is unsafe and impl isn't unsafe
-impl Bar for Foo { }
-// this will compile
-unsafe impl Bar for Foo { }
+unsafe impl Bar for Foo { } // ok!
 ```