Fix doc tests

Author: brson

src/doc/trpl/associated-constants.md

@@ -2,7 +2,7 @@
 
 With the `associated_consts` feature, you can define constants like this:
 
-```rust
+```rust,ignore
 #![feature(associated_consts)]
 
 trait Foo {
@@ -41,7 +41,7 @@ error: not all trait items implemented, missing: `ID` [E0046]
 
 A default value can be implemented as well:
 
-```rust
+```rust,ignore
 #![feature(associated_consts)]
 
 trait Foo {
@@ -68,7 +68,7 @@ add our own definition.
 Associated constants don’t have to be associated with a trait. An `impl` block
 for a `struct` works fine too:
 
-```rust
+```rust,ignore
 #![feature(associated_consts)]
 
 struct Foo;