[README] Fix typo in --foundation-build-dir
[README] Fix typo in --foundation-build-dir

Remove the extra slash. Thanks for the heads up, @hughbe!
tiagomartinho/swift-corelibs-xctest,apple/swift-corelibs-xctest,apple/swift-corelibs-xctest,yoonapps/swift-corelibs-xctest,yoonapps/swift-corelibs-xctest,tiagomartinho/swift-corelibs-xctest