fix a typo in the example code in the doc comments of Ipv6Addr::to_bits()

Author: hltj

library/core/src/net/ip_addr.rs

@@ -1294,7 +1294,7 @@ impl Ipv6Addr {
     ///     0x1020, 0x3040, 0x5060, 0x7080,
     ///     0x90A0, 0xB0C0, 0xD0E0, 0xF00D,
     /// );
-    /// assert_eq!(0x102030405060708090A0B0C0D0E0F00D_u128, u128::from(addr));
+    /// assert_eq!(0x102030405060708090A0B0C0D0E0F00D_u128, addr.to_bits());
     /// ```
     ///
     /// ```