Improve example in the 'arrays' concept and connected exercise (#572)

* Update introduction.md

* Update about.md

* Update introduction.md

concepts/arrays/about.md

@@ -44,9 +44,7 @@ Access, assign, append values using the index operator:
 ```php
 $prime_numbers = [1, 3, 5, 7, 11, 12];
 
-$prime_numbers[5] = 13;
-
-$unlucky_number = $arr[3]; // replace 12 with 13
+$prime_numbers[5] = 13; // replace 12 with 13
 
 $prime_numbers[] = 17; // array now contains [1, 3, 5, 7, 11, 13, 17]
 ```

concepts/arrays/introduction.md

@@ -25,9 +25,7 @@ Access, assign, append values using the index operator:
 ```php
 $prime_numbers = [1, 3, 5, 7, 11, 12];
 
-$prime_numbers[5] = 13;
-
-$unlucky_number = $arr[3]; // replace 12 with 13
+$prime_numbers[5] = 13; // replace 12 with 13
 
 $prime_numbers[] = 17; // array now contains [1, 3, 5, 7, 11, 13, 17]
 ```

exercises/concept/language-list/.docs/introduction.md

@@ -27,9 +27,7 @@ Access, assign, append values using the index operator:
 ```php
 $prime_numbers = [1, 3, 5, 7, 11, 12];
 
-$prime_numbers[5] = 13;
-
-$unlucky_number = $arr[3]; // replace 12 with 13
+$prime_numbers[5] = 13; // replace 12 with 13
 
 $prime_numbers[] = 17; // array now contains [1, 3, 5, 7, 11, 13, 17]
 ```