[CDEC-326] stack.yaml: fix invalid ref for haskell-hedgehog
[CDEC-326] stack.yaml: fix invalid ref for haskell-hedgehog

The commit 2c9e51804e8217dff89f5c32cbe0d79ce20bc508 does not exist.

(cherry picked from commit 063f51996867cf659b5e072b1ae19c9060fa2082)

input-output-hk/pos-haskell-prototype,input-output-hk/cardano-sl,input-output-hk/cardano-sl,input-output-hk/cardano-sl,input-output-hk/pos-haskell-prototype,input-output-hk/pos-haskell-prototype,input-output-hk/cardano-sl