12_calculator: Clarify instructions (#579)

Author: henrylin03

12_calculator/README.md

@@ -2,7 +2,11 @@
 
 The goal for this exercise is to create a calculator that does the following:
 
-add, subtract, get the sum, multiply, get the power, and find the factorial
+1. `add()`: take two numbers and return their sum
+2. `subtract()`: take two numbers and return the result after subtracting the second from the first
+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)
 
-In order to do this 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 file that houses the Jest test cases.
+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.