Update portable pdb guidance in 4.7.1

Unfortunately due to microsoft/dotnet#529 we had to withdraw Portable PDB from 4.7.1 but we anticipate having the feature restored in 4.7.2 with much improved performance.

Documentation/diagnostics/portable_pdb.md

@@ -20,7 +20,7 @@ Windows PDBs can only be written or read on Windows. All Windows tooling support
 Portable PDBs can be read on any operating system, but there are a number of places where they aren't supported yet. Here are a few –
 
 * Older versions of the Visual Studio debugger (versions before VS 2015 Update 2)
-* .NET Framework prior 4.7.1: printing stack traces with mappings back to line numbers (such as in an ASP.NET error page). The name of methods is unaffected, only the source file names and line numbers are unsupported.
+* .NET Framework prior 4.7.2: printing stack traces with mappings back to line numbers (such as in an ASP.NET error page). The name of methods is unaffected, only the source file names and line numbers are unsupported.
 * C# Code analysis (aka FxCop), note that this doesn't apply to Roslyn Analyzer
 * Symbol server (ex: SymbolsSource.org)
 * Running post-compilation build step that consumes or modifies the PDB using older versions of tools such as CCI, CodeContracts.