Fix "SHFB(0,0): error BE0071: Unable to locate information for the project framework version '.NET Framework 4.5' or a suitable redirected version on this system. See error number help topic for details." (note: generates a new warning with auto-upgrade to net5.0, this is intentional)

Documentation/ICSharpCode.AvalonEdit.Documentation.shfbproj

@@ -13,7 +13,7 @@
     <RootNamespace>Documentation</RootNamespace>
     <Name>Documentation</Name>
     <!-- SHFB properties -->
-    <FrameworkVersion>.NET Framework 4.5</FrameworkVersion>
+    <FrameworkVersion>.NET Framework 4.7.2</FrameworkVersion>
     <OutputPath>.\Help\</OutputPath>
     <HtmlHelpName>AvalonEdit Documentation</HtmlHelpName>
     <Language>en-US</Language>