Fix document example

[sizumita]

• Make send mutable because of fixes rust error.
• Fix typo

[sizumita]

and I think

let (mut send, recv) = connection
        .open_bi()
        .await?;

is

let (mut send, recv) = connection.connection
        .open_bi()
        .await?;

[djc]

Thanks! Would you like to submit another PR for the connection.connection issue?