Fix error code example

compiler/rustc_error_codes/src/error_codes/E0232.md

@@ -1,19 +1,14 @@
 The `#[rustc_on_unimplemented]` attribute lets you specify a custom error
 message for when a particular trait isn't implemented on a type placed in a
-position that needs that trait. For example, when the following code is
-compiled:
+position that needs that trait. The attribute will let you filter on
+various types, with `on`:
 
 ```compile_fail,E0232
 #![feature(rustc_attrs)]
 #![allow(internal_features)]
 
-#[rustc_on_unimplemented(lorem="")] // error!
+#[rustc_on_unimplemented(on(blah, message = "foo"))] // error!
 trait BadAnnotation {}
 ```
-
-there will be an error about `bool` not implementing `Index<u8>`, followed by a
-note saying "the type `bool` cannot be indexed by `u8`".
-
-For this to work, some note must be specified. An empty attribute will not do
-anything, please remove the attribute or add some helpful note for users of the
-trait.
+For this to work a cfg-like predicate must be supplied. A malformed filter
+will not do anything.