freeze: fix cmath.pyx when run as __main__

Demos/freeze/cmath.pyx

@@ -20,5 +20,5 @@ if __name__ == "__main__":
     if len(sys.argv) != 2:
         sys.stderr.write("USAGE: %s n\nPrints n!.\n" % sys.argv[0])
         sys.exit(1)
-    n = map(float, sys.argv[1:])
+    n, = map(float, sys.argv[1:])
     print factorial(n)