fix description

Author: soulomoon

ghcide/src/Development/IDE/Core/Shake.hs

@@ -1357,8 +1357,9 @@ updatePositionMapping IdeState{shakeExtras = ShakeExtras{positionMapping}} Versi
                 -- Very important to use mapAccum here so that the tails of
                 -- each mapping can be shared, otherwise quadratic space is
                 -- used which is evident in long running sessions.
-                -- mapAccumWithKey is the one with smaller key accumulate in acc
-                EM.mapAccumWithKey (\acc _k (delta, _) -> let new = addOldDelta delta acc in (new, (delta, acc)))
+                -- mapAccumWithKey is the one with smaller keys accumulated in acc
+                -- so smaller keys can be shared.
+                EM.mapAccumWithKey (\acc _k (delta, _) -> let new = addDelta delta acc in (new, (delta, acc)))
                   zeroMapping
                   (EM.insert _version (shared_change, zeroMapping) mappingForUri)
         shared_change = mkDelta changes