fdo#72995: Changed label to 'Formula cell must contain a formula.'

Change-Id: Iaa8528985935549f0ef1bdc96edfbde7d9b104c9
Reviewed-on: https://gerrit.libreoffice.org/7234
Reviewed-by: Kohei Yoshida <libreoffice@kohei.us>
Tested-by: Kohei Yoshida <libreoffice@kohei.us>