Update help for X error bars

Change-Id: I732574ad89d5341fb9e8cd988b342bc9daa80191
Reviewed-on: https://gerrit.libreoffice.org/7262
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>
Tested-by: Markus Mohrhard <markus.mohrhard@googlemail.com>