fix: clarify subject of declarations.js

subjects/declarations/README.md

@@ -15,7 +15,7 @@ Create the following constant variables:
   - arr: an array of the 3 values: 4, undefined, '2'
   - obj: an object with 3 property (str, num & bool)
 
-`nested` and `obj` must be frozen to prevent changes from them.
+`nested`, `arr` and `obj` must be frozen to prevent changes from them.
 
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