Make `_typeReaderTypeInfo` static

Seems I missed this originally, whoops.

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

@@ -7,7 +7,7 @@ namespace Discord.Commands
     [AttributeUsage(AttributeTargets.Parameter)]
     public class OverrideTypeReaderAttribute : Attribute
     {
-        private readonly TypeInfo _typeReaderTypeInfo = typeof(TypeReader).GetTypeInfo();
+        private static readonly TypeInfo _typeReaderTypeInfo = typeof(TypeReader).GetTypeInfo();
 
         public Type TypeReader { get; }