OSUpdateNotifier: ForceUpdateStartTimeInEpoch value set syntax

code in line 970 need correct syntax
current: 
```
# Determine the current start time in epoch seconds for forced update via GUI
ForceUpdateStartTimeInEpoch="$(/bin/date -jf "%a %b %d %T %Z %Y" "$(/bin/date)" +"%s")
```

Fix:
```
# Determine the current start time in epoch seconds for forced update via GUI
ForceUpdateStartTimeInEpoch="$(/bin/date +"%s")
```"