typo fix (#52)

(cherry picked from commit a1a7925)

source/data-formats/codecs.txt

@@ -339,7 +339,7 @@ corresponding ``Codec`` to handle encoding and decoding:
   kelvins as an ``Int`` value. For this field, use the ``IntegerCodec``
   provided in the BSON library.
 
-The following code shows how to can implement a ``Codec`` for the
+The following code shows how to implement a ``Codec`` for the
 ``Monolight`` class. The constructor expects an instance of
 ``CodecRegistry`` from which it retrieves the ``Codec`` instances needed
 to encode and decode the class fields: