fixed sh syntax error

https://forums.whonix.org/t/qvm-run-fails-in-whonix-vms

Thanks to entr0py for the bug report!

qubes-rpc/qvm-run

@@ -20,7 +20,7 @@
 #
 #
 
-function print_usage(){
+print_usage() {
 cat >&2 <<USAGE
 Usage: $0 vmname command arguments
 Executes a command in another VM using the qubes.VMShell RPC service.  The