update unstable book and error example

Author: leoyvens

src/doc/unstable-book/src/language-features/optin-builtin-traits.md

@@ -24,9 +24,7 @@ Example:
 ```rust
 #![feature(optin_builtin_traits)]
 
-trait Valid {}
-
-impl Valid for .. {}
+auto trait Valid {}
 
 struct True;
 struct False;

src/librustc_typeck/diagnostics.rs

@@ -1854,7 +1854,7 @@ unsafe impl !Clone for Foo { }
 
 This will compile:
 
-```
+```ignore (ignore auto_trait future compatibility warning)
 #![feature(optin_builtin_traits)]
 
 struct Foo;