Use Pin<Box<>> for collation future (#1792)

polkadot/roadmap/implementers-guide/src/node/collators/collation-generation.md

@@ -39,7 +39,7 @@ struct CollationGenerationConfig {
   /// Collate will be called with the relay chain hash the parachain should build 
   /// a block on and the `ValidationData` that provides information about the state
   /// of the parachain on the relay chain.
-  collator: Box<dyn Fn(Hash, &ValidationData) -> Box<dyn Future<Output = Option<Collation>>>>
+  collator: Box<dyn Fn(Hash, &ValidationData) -> Pin<Box<dyn Future<Output = Option<Collation>>>>>
   para_id: ParaId,
 }
 ```