Missing async from getCollator causes type mismatch

Signed-off-by: Taras Mankovski <taras@frontside.com>

docs/features/search/how-to-guides.md

@@ -219,7 +219,7 @@ export class YourCollatorFactory implements DocumentCollatorFactory {
       };
     }
   }
-  getCollator() {
+  async getCollator() {
     return Readable.from(this.execute());
   }
 }