query: fix undefined variable name
query: fix undefined variable name

* Fixes typo in `page_` variable name. (closes #25)

Signed-off-by: Jiri Kuncar <2bdffcb416fed3002465740d19a053a39918cbe5@cern.ch>

inveniosoftware/invenio-oaiserver,inveniosoftware/invenio-oaiserver