Update the link to the file Portable PDB v1.0: Format Specification (#77911)

Author: kirisky

docs/specs/PortablePdb-Metadata.md

@@ -1,4 +1,4 @@
 # Portable PDB v1.0: Format Specification
 
-Moved to 
-https://github.com/dotnet/corefx/blob/main/src/System.Reflection.Metadata/specs/PortablePdb-Metadata.md
+Moved to
+https://github.com/dotnet/runtime/blob/main/docs/design/specs/PortablePdb-Metadata.md