Fix documentation on func (Term) Exec
Fix documentation on func (Term) Exec

The documentation's example expects a result to be returned. Exec only responds with an error.
dancannon/gorethink,GoRethink/gorethink,SKatiyar/gorethink,GoRethink/gorethink,dancannon/gorethink,GoRethink/gorethink,brendanobrienesq/gorethink,dancannon/gorethink