add scheme to URLs in NonBlockingFileIO docs (#460)
add scheme to URLs in NonBlockingFileIO docs (#460)

Motivation:

In the NonBlockingFileIO, the docs contain links but the scheme was
missing so the links didn't work.
Note: Both URLs don't work with https:// so I had to use http://,
sorry...

Modifications:

added a scheme to the URLs so they actually work

Result:

dead links in NonBlockingFileIO fixed
apple/swift-nio,apple/swift-nio,apple/swift-nio,apple/swift-nio,apple/swift-nio