Made button caption in bib entry dialog clearer

Change-Id: Ib917865211407b9aed1cb04eaec9b71c63ead182
Reviewed-on: https://gerrit.libreoffice.org/7179
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>