docs: Correct the example for "Starting a server" in the API reference

[xuanzhi33]

It seems that server.close() is not an async function, so I changed

await server.close()

to

server.close()
await server.wait_closed()

in the API reference.

[aaugustin]

Thank for you the fix — indeed close() isn't a coroutine: https://github.com/python-websockets/websockets/blob/e217458ef8b692e45ca6f66c5aeb7fad0aee97ee/src/websockets/legacy/server.py#L724

I'm going to check how I introduced this mistake and merge the fix.

[xuanzhi33]

I found the same mistake in the FAQ and fixed it.