Fix broken link for language-versioning spec (dart-lang/package_config#120)

Thanks!
And yes, it's an issue that files do not have permalinks, but we also do want to use the hierarchical structure of the repo for placing them where they belong, and we don't want to keep copies around, because that risks older copies being stale.

No good solution found. (If only you could tag files and refer to them by tag.)