[README] Fix "swift" capitalization
[README] Fix "swift" capitalization

A capitalized "Swift" matches usage in the rest of the README.

tiagomartinho/swift-corelibs-xctest,tiagomartinho/swift-corelibs-xctest,apple/swift-corelibs-xctest,apple/swift-corelibs-xctest,yoonapps/swift-corelibs-xctest,yoonapps/swift-corelibs-xctest