Update Collatz-conjecture error wording (exercism#2859)

Author: resu-xuniL

exercises/practice/collatz-conjecture/.docs/instructions.append.md

@@ -7,5 +7,5 @@ If `n` is not a positive integer, stop the program from being executed further a
 In JavaScript, this can be done using the `throw` statement.
 
 ```javascript
-throw new Error('Only positive numbers are allowed');
+throw new Error('Only positive integers are allowed');
 ```