Fix fdo#80444: Help pages fo STDEV functions

help page for STDEV.P and STDEV.S

Change-Id: Iab0cd53f0cdda84e1331a204c74f08c154e9589b
Reviewed-on: https://gerrit.libreoffice.org/10568
Reviewed-by: David Tardon <dtardon@redhat.com>
Tested-by: David Tardon <dtardon@redhat.com>