Syntax error in error message

pysimpledmx.py, line 47 should be:

`print 'Invalid channel specified: %s' % (chan-1)`

Note the parens around `chan-1`.  Without them, the interpreter tries to subtract 1 from the string "Invalid..."

I've tried submitting the changes but am getting a 403 error on push (also: I'm not super familiar with pushing to GitHub repos that I don't own, which may have something to do with that).