Fix incompatible package reference for OSX

Author: AlexNDRmac

src/ImageSharp/ImageSharp.csproj

@@ -32,7 +32,7 @@
   <ItemGroup Condition="$(TargetFramework.StartsWith('netcoreapp')) AND $([MSBuild]::IsOSPlatform('OSX'))">
     <PackageReference Include="runtime.osx.10.10-x64.CoreCompat.System.Drawing" Version="5.8.64" />
   </ItemGroup>
-  <ItemGroup Condition="$(TargetFramework.StartsWith('netstandard')) AND $([MSBuild]::IsOSPlatform('OSX'))">
+  <ItemGroup Condition="$(TargetFramework.StartsWith('netstandard2')) AND $([MSBuild]::IsOSPlatform('OSX'))">
     <PackageReference Include="runtime.osx.10.10-x64.CoreCompat.System.Drawing" Version="5.8.64" />
   </ItemGroup>