Factorial: Link to simpler resource (#644)

Author: dantoms

foundations/12_calculator/README.md

@@ -7,6 +7,6 @@ The goal for this exercise is to create a calculator that does the following:
 3. `sum()`: take an array of numbers and add all the numbers together
 4. `multiply()`: take an array of numbers and multiply all the numbers together
 5. `power()`: take two numbers (base and exponent) and return the result of raising the base to that exponent
-6. `factorial()`: given a single number, find its [factorial](https://en.wikipedia.org/wiki/Factorial)
+6. `factorial()`: given a single number, find its [factorial](https://www.mathsisfun.com/numbers/factorial.html)
 
 Please fill out each function with your solution. Make sure to `return` the value so you can test it in Jest! To see the expected value, take a look at the `.spec.js` file that houses the Jest test cases.