Fix link to portable pdb spec (#4569)
Fix link to portable pdb spec (#4569)

* Fix link to portable pdb spec

* Update Documentation/diagnostics/portable_pdb.md

Co-Authored-By: Maira Wenzel <034155cd61a6bd91628271f7ab4d545852f363e6@microsoft.com>

Co-authored-by: Maira Wenzel <034155cd61a6bd91628271f7ab4d545852f363e6@microsoft.com>
dotnet/core