Fix clashing exercise numbers (#596)

Stale numbering - missed #509

Author: mao-sz

13_factorial/README.md renamed to 17_factorial/README.md

@@ -1,4 +1,4 @@
-# Exercise 13 - Factorial
+# Exercise 17 - Factorial
 
 Write a recursive [factorial](https://simple.wikipedia.org/wiki/Factorial) function that takes a non-negative integer, and returns the product of all positive integers less than or equal to the input integer. An input of `0` should return `1`. The function should only accept numbers, so `'4'` should not be accepted as it is a string. All invalid inputs should return `undefined`.
 

13_factorial/factorial.js renamed to 17_factorial/factorial.js

@@ -1,4 +1,4 @@
-# Exercise 14 - contains
+# Exercise 18 - contains
 
 Write a function that searches for a value in a nested object. It returns true if the object contains that value.