Update async usages in README and examples to shutdown properly

[michaelmcguire]

Motivation:

In several examples of async/await support, the call to shutdown() on the HTTPClient are made with await but do not call get() on the EventLoopFuture.

Modifications:

Add the .get() call in four locations that need it.

Result:

After fixing this, the client should properly get closed down and there are no longer warnings emitted by the compiler.

[dnadoba]

@swift-server-bot test this please