minor grammar fix

Author: donkirkby

docs/api-guide/relations.md

@@ -273,7 +273,7 @@ If you want to implement a read-write relational field, you must also implement
 
 ## Example
 
-For, example, we could define a relational field, to serialize a track to a custom string representation, using its ordering, title, and duration.
+For example, we could define a relational field to serialize a track to a custom string representation, using its ordering, title, and duration.
 
     import time