Documentation errors

There're a few issues with the example server, even after using the prerelease version. Probably not worth a PR, so here's an update example with notes below:

```python
import asyncio

from echo import EchoBase, EchoResponse, EchoStreamResponse
from grpclib.server import Server
from typing import AsyncIterator

class EchoService(EchoBase):
async def echo(self, value: str, extra_times: int) -> EchoResponse:
return EchoResponse(values=[value] * extra_times)

async def echo_stream(
self, value: str, extra_times: int
) -> AsyncIterator[EchoStreamResponse]:
for _ in range(extra_times):
yield EchoStreamResponse(value=value)

async def start_server():
HOST = "127.0.0.1"
PORT = 50051
server = Server([EchoService()])
await server.start(HOST, PORT)
await server.wait_closed()

if __name__ == "__main__":
loop = asyncio.get_event_loop()
loop.run_until_complete(start_server())
loop.close()

```

### Changes

- Added imports of `EchoReponse` and `EchoStreamResponse`
- Updated `echo` and `echo_stream` to return the correct types
- Updated `PORT` to match that in the client example
- Replaced `await server.run_forever()` (raises a `NotImplementedError`) with `await server.wait_closed()`
- Added an asyncio loop at the bottom to actually run the server

Start with the example server documentation and compare its code with the client example. Update the imports, response types, port, server shutdown call, and asyncio entry point as listed; done means the example matches the working server behavior and can be run without the reported errors.

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