fdo#62879 Changed wording of protected section dialog

Clarified the wording of protected section dialog from
"Readonly content cannot be changed." to "Write protected sections cannot be
changed."

Change-Id: Ic6df2d84b6cc9e87727306eff8aa0e06895cab17
Reviewed-on: https://gerrit.libreoffice.org/9905
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>