Disambiguate where the suggested 'force' function comes from

Follow-up to PR haskell#231.

Users could easily be misled that it's `Control.DeepSeq.force`, not `Data.Vector.force`,
and the former does not fix the issue at all.

Data/Vector/Mutable.hs

@@ -193,7 +193,7 @@ loopM !n k = let
   in go 0
 
 uninitialised :: a
-uninitialised = error "Data.Vector.Mutable: uninitialised element. If you are trying to compact a vector, use the 'force' function to remove uninitialised elements from the underlying array."
+uninitialised = error "Data.Vector.Mutable: uninitialised element. If you are trying to compact a vector, use the 'Data.Vector.force' function to remove uninitialised elements from the underlying array."
 
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