Update instructions.md (#1618)

Clarifies the final point, which is wrong written.

(Thanks to https://forum.exercism.org/t/final-task-for-boutique-inventory-improvements/8906 for the nudge)

Author: iHiD

exercises/concept/boutique-inventory-improvements/.docs/instructions.md

@@ -43,7 +43,7 @@ BoutiqueInventory.new([
 
 # 2. Refactor `total_stock`
 
-Refactor `total_stock` to use the new block shortcut you've learnt rather than hashes.
+Refactor `total_stock` to use the openstruct's method, rather than referencing a hash.
 As a reminder, the method should return::
 
 ```ruby