Merge pull request #581 from alanz/ghc-8.4.2-haddock

Use working haddock libraries for GHC 8.4.2

Author: alanz

stack.yaml

@@ -26,8 +26,8 @@ extra-deps:
 - apply-refact-0.5.0.0
 - brittany-0.11.0.0
 - constrained-dynamic-0.1.0.0
-- haddock-api-2.19.0.1
-- haddock-library-1.5.0.1
+- haddock-api-2.20.0
+- haddock-library-1.6.0
 - haskell-lsp-0.2.2.0
 - haskell-lsp-types-0.2.2.0
 - syz-0.2.0.0