Fix typo in "d9d48e8 qrexec: use tray notification when..."

Fixes QubesOS/qubes-issues#1446

Thanks HW42 for the report.

(cherry picked from commit fa8ebeb42d)

qrexec/qrexec-policy

@@ -102,7 +102,7 @@ def spawn_target_if_necessary(vm):
     # use qvm-run instead of vm.start() to make sure that nothing is written
     # to stdout and nothing is read from stdin
     null = open("/dev/null", "r+")
-    subprocess.call(["qvm-run", "-a", "-tray", "-q", vm.name, "true"],
+    subprocess.call(["qvm-run", "-a", "--tray", "-q", vm.name, "true"],
                     stdin=null, stdout=null)
     null.close()