Add missing -command argument in powershell call

[MagicalDrizzle]

it seems like at least with powershell core - you need the -command argument passed to powershell to execute a built-in function.
currently, the backup filename is not prefs-backup-(date/time).js but prefs-backup-All parameters are case-insensitive.js, because this is the output of the powershell command

The argument 'get-date' is not recognized as the name of a script file. Check the spelling of the name, or if a path was included, verify that the path is correct and try again.
...
All parameters are case-insensitive.

[strotee]

Pro tip: -command allows one to shorten it by writing -c.

Edit: Same thing with -format can be written as -f.

[earthlng]

thanks @MagicalDrizzle