Fix framework version for doc project

Author: thomaslevesque

docs/Documentation.shfbproj

@@ -14,7 +14,7 @@
     <RootNamespace>Documentation</RootNamespace>
     <Name>Documentation</Name>
     <!-- SHFB properties -->
-    <FrameworkVersion>Cross-platform (.NET Core/.NET Standard)</FrameworkVersion>
+    <FrameworkVersion>.NET Core/.NET Standard/.NET 5.0+</FrameworkVersion>
     <OutputPath>api</OutputPath>
     <HtmlHelpName>Linq.Extras</HtmlHelpName>
     <Language>en-US</Language>