correct XSheetAnnotations GetByIndex test for #fdo#80551

the previous testGetCount test already added a note on sheet 2
testGetByIndex now works on sheet 3

Change-Id: I2cd49aa2bbf5c1b707287518e0f15088f3ee5ec0
Reviewed-on: https://gerrit.libreoffice.org/9938
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>