Minor fix of MonadFail instance for `ReadPrec`

This fixes the instance to be semantically cleaner but other than that
this fix has no practical consequences as the implementations of
`Monad(fail)` and `MonadFail(fail)` for `ReadP` coincide.

libraries/base/Text/ParserCombinators/ReadPrec.hs

@@ -85,7 +85,7 @@ instance Monad ReadPrec where
   P f >>= k = P (\n -> do a <- f n; let P f' = k a in f' n)
 
 instance MonadFail.MonadFail ReadPrec where
-  fail s    = P (\_ -> fail s)
+  fail s    = P (\_ -> MonadFail.fail s)
 
 instance MonadPlus ReadPrec