Fix(06_Days_Loop): expression error

There is a mistake in 'for of loop' section:

06_Day_Loops/06_day_loops.md

@@ -178,7 +178,7 @@ for (const num of numbers) {
 // adding all the numbers in the array
 let sum = 0
 for (const num of numbers) {
-  sum += sum + num  // can be also shorten like this, sum += num
+  sum = sum + num  // can be also shorten like this, sum += num
 }
 console.log(sum) // 15