Unavailble link to Portable PDB v1.0: Format Specification

[kirisky]

The link to Portable PDB v1.0: Format Specification is no longer working as the dotnet/corefx repo has been merged into dotnet/runtime

Should update the broken link to the new location:
https://github.com/dotnet/runtime/blob/main/docs/design/specs/PortablePdb-Metadata.md

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