user guide has dead link to MonadFailProposal on prime.haskell.org

Summary

Location of documentation issue: GHC user guide

https://downloads.haskell.org/ghc/latest/docs/html/users_guide/using-warnings.html?highlight=monadfail#ghc-flag--Wmissing-monadfail-instances points to https://prime.haskell.org/wiki/Libraries/Proposals/MonadFail which does not seem to exist.

When I search for "MonadFail Proposal", the top result is https://wiki.haskell.org/MonadFail_Proposal which exists, but says it's been moved to the non-existing page.

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