Fix typo in ConnectionPool and AsyncConnectionPool

[n-thumann]

Summary

This PR fixes a typo in the ConnectionPool and AsyncConnectionPool classes.

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.

[tomchristie]

Oh great, thanks.
And you've nudged a flaky test as well.
I wonder what that's about?

[tomchristie]

Bumped into a flaky test...

______________ test_connection_pool_timeout_during_request[trio] _______________

    @pytest.mark.anyio
    async def test_connection_pool_timeout_during_request():
        """
        An async timeout when writing an HTTP/1.1 response on the connection pool
        should leave the pool in a consistent state.
    
        In this case, that means the connection will become closed, and no
        longer remain in the pool.
        """
        network_backend = SlowWriteBackend()
        async with httpcore.AsyncConnectionPool(network_backend=network_backend) as pool:
            with anyio.move_on_after(0.01):
                await pool.request("GET", "http://example.com/")
>           assert not pool.connections
E           AssertionError: assert not [<AsyncHTTPConnection ['[http://example.com:80](http://example.com/)', HTTP/1.1, NEW, Request Count: 0]>]
E            +  where [<AsyncHTTPConnection ['[http://example.com:80](http://example.com/)', HTTP/1.1, NEW, Request Count: 0]>] = <AsyncConnectionPool [Requests: 0 active, 0 queued | Connections: 1 active, 0 idle]>.connections

tests/test_cancellations.py:111: AssertionError