Replace Nullable package with PolySharp

Author: atifaziz

src/DocoptNet/DocoptNet.csproj

@@ -80,7 +80,7 @@ Portions Copyright (C) West Wind Technologies, 2008.
       <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
       <PrivateAssets>all</PrivateAssets>
     </PackageReference>
-    <PackageReference Include="Nullable" Version="1.3.0">
+    <PackageReference Include="PolySharp" Version="1.8.1">
       <PrivateAssets>all</PrivateAssets>
       <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
     </PackageReference>