fixes typo in exers for ch9
fixes typo in exers for ch9

raventid/coursera_learning,raventid/coursera_learning,raventid/coursera_learning,raventid/coursera_learning,raventid/coursera_learning,raventid/coursera_learning,raventid/coursera_learning,raventid/coursera_learning,raventid/coursera_learning,raventid/coursera_learning