Update Discord.Net.Core.csproj (#1081)

src/Discord.Net.Core/Discord.Net.Core.csproj

@@ -9,7 +9,7 @@
   </PropertyGroup>
   <ItemGroup>
     <PackageReference Include="Newtonsoft.Json" Version="11.0.2" />
-    <PackageReference Include="System.Collections.Immutable" Version="1.4.0" />
+    <PackageReference Include="System.Collections.Immutable" Version="1.3.1" />
     <PackageReference Include="System.Interactive.Async" Version="3.1.1" />
   </ItemGroup>
 </Project>