docs: update CategoryId remarks for detaching a channel

setting a categoryid to 0 will result in a bad request, null will actually detach

src/Discord.Net.Core/Entities/Channels/NestedChannelProperties.cs

@@ -10,7 +10,7 @@ namespace Discord
         /// </summary>
         /// <remarks>
         ///     Setting this value to a category's snowflake identifier will change or set this channel's parent to the
-        ///     specified channel; setting this value to <c>0</c> will detach this channel from its parent if one
+        ///     specified channel; setting this value to <c>null</c> will detach this channel from its parent if one
         ///     is set.
         /// </remarks>
         public Optional<ulong?> CategoryId { get; set; }