change null-coalescing to nil-coalescing
change null-coalescing to nil-coalescing

Docs: https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift_Programming_Language/BasicOperators.html#//apple_ref/doc/uid/TP40014097-CH6-ID72
jawwad/swift-algorithm-club,tkester/swift-algorithm-club,daisysomus/swift-algorithm-club,tomtclai/swift-algorithm-club,daisysomus/swift-algorithm-club,jawwad/swift-algorithm-club,tomtclai/swift-algorithm-club,uteschiehlen/swift-algorithm-club,TheIronBorn/swift-algorithm-club,BenEmdon/swift-algorithm-club,uteschiehlen/swift-algorithm-club,hollance/swift-algorithm-club,BenEmdon/swift-algorithm-club,aatalyk/swift-algorithm-club,TheIronBorn/swift-algorithm-club,jawwad/swift-algorithm-club,tkester/swift-algorithm-club,eugenepavlyuk/swift-algorithm-club,raywenderlich/swift-algorithm-club,boqian2000/swift-algorithm-club,boqian2000/swift-algorithm-club,raywenderlich/swift-algorithm-club,raywenderlich/swift-algorithm-club,aatalyk/swift-algorithm-club,eugenepavlyuk/swift-algorithm-club