docs: point intersphinx to Flask 0.12
docs: point intersphinx to Flask 0.12

Signed-off-by: Dinos Kousidis <2232407337c30afa275b4ef42b0b26111a2f9dfc@cern.ch>

inveniosoftware/invenio-oaiserver,inveniosoftware/invenio-oaiserver