admin: fix of flask_admin import statement
admin: fix of flask_admin import statement

Signed-off-by: Jacopo Notarstefano <9b2cfbb82b98c608f2c886ba9b6f43b79f78200a@cern.ch>

tiborsimko/invenio-oauthclient,inveniosoftware/invenio-oauthclient,inveniosoftware/invenio-oauthclient,tiborsimko/invenio-oauthclient,inveniosoftware/invenio-oauthclient,tiborsimko/invenio-oauthclient