fix: PacakgeReference belongs under an ItemGroup

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

@@ -11,8 +11,8 @@
     <PackageReference Include="Newtonsoft.Json" Version="11.0.2" />
     <PackageReference Include="System.Collections.Immutable" Version="1.3.1" />
     <PackageReference Include="System.Interactive.Async" Version="3.1.1" />
+    <PackageReference Include="IDisposableAnalyzers" Version="2.1.2">
+      <PrivateAssets>all</PrivateAssets>
+    </PackageReference>
   </ItemGroup>
-  <PackageReference Include="IDisposableAnalyzers" Version="2.1.2">
-    <PrivateAssets>all</PrivateAssets>
-  </PackageReference>
 </Project>