Remove unnecessary mut from udp doctests

library/std/src/net/udp.rs

@@ -39,7 +39,7 @@ use crate::time::Duration;
 ///
 /// fn main() -> std::io::Result<()> {
 ///     {
-///         let mut socket = UdpSocket::bind("127.0.0.1:34254")?;
+///         let socket = UdpSocket::bind("127.0.0.1:34254")?;
 ///
 ///         // Receives a single datagram message on the socket. If `buf` is too small to hold
 ///         // the message, it will be cut off.