feat(more-or-less): improve subject

- Clarify the behavior of the required functions

subjects/more-or-less/README.md

@@ -3,11 +3,14 @@
 ### Instructions
 
 Create 4 functions:
+
 - `more` that takes 1 argument and adds 1 to it.
 - `less` that takes 1 argument and subtracts 1 from it.
 - `add` that takes 2 arguments and adds them together.
 - `sub` that takes 2 arguments and subtracts the second argument from the first.
 
+All the functions must return the result of the operations specified above.
+
 ### Notions
 
 - [Functions](https://nan-academy.github.io/js-training/examples/functions.js)