docs(numMax): fix the readme

subjects/mobile-dev/max-num/README.md

@@ -2,4 +2,24 @@
 
 ### Instructions
 
-Write a function named `maxNum`, which accepts `int` parameters, and returns the maximum number of the three.
+Your task is to implement a function `maxNum`, which should accept three integer parameters and return the maximum of the three.
+
+### Usage
+
+```dart
+void main() {
+  print(maxNum(1, 2, 3));
+  print(maxNum(0, 0, 0));
+  print(maxNum(-1, -5, 0));
+}
+```
+
+And its output :
+
+```console
+$ dart test.dart
+3
+0
+0
+$
+```