Fix non-canonical target framework name

This was the root cause behind dotnet/project-system#2388

Author: AArnott

src/Directory.Build.props

@@ -5,7 +5,7 @@
     <OutputPath>$(MSBuildThisFileDirectory)..\bin\$(Configuration)\</OutputPath>
 
     <DesktopOnlyFrameworks>net45;net40;net20</DesktopOnlyFrameworks>
-    <PortableOnlyFrameworks>netstandard1.1;portable-net4+win8+wpa81;portable-net45+win8+wpa81</PortableOnlyFrameworks>
+    <PortableOnlyFrameworks>netstandard1.1;portable-net40+win8+wpa81;portable-net45+win8+wpa81</PortableOnlyFrameworks>
     <DesktopAndPortableFrameworks>$(PortableOnlyFrameworks);$(DesktopOnlyFrameworks)</DesktopAndPortableFrameworks>
 
     <GenerateDocumentationFile>true</GenerateDocumentationFile>