Fixed minor issue in compound documents example
Fixed minor issue in compound documents example

Per @v-ken's comment, this commit fixes a small error in the documentation of
the compound documents (i.e. `included`) feature in
PR marshmallow-code/marshmallow-jsonapi#40

https://github.com/marshmallow-code/marshmallow-jsonapi/pull/40#issuecomment-225129348

akira-dev/marshmallow-jsonapi,marshmallow-code/marshmallow-jsonapi