Add mention of "update" command to scripts/app help

[markshep]

The scripts/app shell script has an update command, but that wasn't mentioned in the help. Commit ab4ebf5703146487e962705adc0893163f3dbdf1 adds that command to the help message.

Also while reading through the script I noticed that the bit which checks that an app has been specified on the commmand line (for all commands except ls-installed) uses an if/else, but because the if clause ends in an exit there's no need for the else clause. Commit ddbce830ceb9543023d2dcf6d0db0c1ed043f1c3 tidies that up.