auto merge of #12361 : sfackler/rust/rustdoc-test-extern, r=alexcrichton

diff --cc src/libextra/json.rs
index edf5bb63b300421eebb3f153beaaba0555d6ef48,12b632559589c46fea54ed26c5ed875af8a6e5c1..5043cad6f200cca9bb1d174584d0c53bc2f63684
--- 1/src/libextra/json.rs
--- 2/src/libextra/json.rs
+++ b/src/libextra/json.rs
@@@ -125,9 -127,10 +127,10 @@@ fn main() 
  }
  ```
  
 -To decode a json string using `Decodable` trait :
 +To decode a JSON string using `Decodable` trait :
  
  ```rust
+ extern crate extra;
  extern crate serialize;
  use serialize::Decodable;