Update arrays-and-slices.md (#830)

Adjust to correct grammar

Author: ddrouin

arrays-and-slices.md

@@ -5,7 +5,7 @@
 Arrays allow you to store multiple elements of the same type in a variable in
 a particular order.
 
-When you have an array, it is very common to have to iterate over them. So let's
+When you have arrays, it is very common to have to iterate over them. So let's
 use [our new-found knowledge of `for`](iteration.md) to make a `Sum` function. `Sum` will
 take an array of numbers and return the total.