Fix incorrect null xmldocs string

src/Discord.Net.Commands/Attributes/PreconditionAttribute.cs

@@ -8,7 +8,7 @@ namespace Discord.Commands
     {
         /// <summary>
         /// Specify a group that this precondition belongs to. Preconditions of the same group require only one
-        /// of the preconditions to pass in order to be successful (A || B). Specifying <see cref="Group"/> = <see cref="null"/>
+        /// of the preconditions to pass in order to be successful (A || B). Specifying <see cref="Group"/> = <see langword="null"/>
         /// or not at all will require *all* preconditions to pass, just like normal (A &amp;&amp; B).
         /// </summary>
         public string Group { get; set; } = null;