Updated to work with VS2019 and .NET Core 3.1 (#1361)

Author: tig

demos/PuppeteerSharpPdfDemo/PuppeteerSharpPdfDemo.csproj

@@ -2,11 +2,11 @@
 
   <PropertyGroup>
     <OutputType>Exe</OutputType>
-    <TargetFramework>netcoreapp2.2</TargetFramework>
+    <TargetFramework>netcoreapp3.1</TargetFramework>
     <LangVersion>7.3</LangVersion>
   </PropertyGroup>
   <ItemGroup>
-    <PackageReference Include="PuppeteerSharp" Version="1.0.1" />
+    <PackageReference Include="PuppeteerSharp" Version="2.0.0" />
   </ItemGroup>
 
 </Project>