Tell msbuild that docs depends on typelevel (#432)

Author: wallymathieu

src/FSharpPlus.Docs/FSharpPlus.Docs.fsproj

@@ -52,11 +52,8 @@
     <Compile Include="Program.fs" />
   </ItemGroup>
   <ItemGroup>
-    <ProjectReference Include="..\..\src\FSharpPlus\FSharpPlus.fsproj">
-      <Name>FSharpPlus</Name>
-      <Project>{4364a3ea-06d8-4493-be44-e22d5370cca1}</Project>
-      <Private>True</Private>
-    </ProjectReference>
+    <ProjectReference Include="..\..\src\FSharpPlus\FSharpPlus.fsproj" />
+    <ProjectReference Include="..\..\src\FSharpPlus.TypeLevel\FSharpPlus.TypeLevel.fsproj" />
     <PackageReference Update="FSharp.Core" Version="4.6.2" />
     <PackageReference Include="MathNet.Numerics.FSharp" Version="4.8.1" />
   </ItemGroup>