Fix div in junit-jupiter-starter-gradle-kotlin (#751)

Just a minor detail not relevant for the unit tests at all, but the previous implementation of the div method would do integer division and then convert to double, e.g. div(1, 2) == 0.0, not 0.5 as expected

Author: UOBW

junit-jupiter-starter-gradle-kotlin/src/main/kotlin/com/example/project/Calculator.kt

@@ -18,7 +18,6 @@ class Calculator {
 
     fun div(a: Int, b: Int): Double {
         assert(b != 0) { "Division by Zero" }
-        return a / b * 1.0
+        return a.toDouble() / b
     }
-
 }