Merge pull request reduxjs#2239 from coryhouse/patch-10

Grammar fix

docs/recipes/reducers/PrerequisiteConcepts.md

@@ -4,7 +4,7 @@
 As described in [Reducers](../../basics/Reducers.md), a Redux reducer function:
 
 - Should have a signature of `(previousState, action) => newState`, similar to the type of function you would pass to [`Array.prototype.reduce(reducer, ?initialValue)`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/Reduce)
-- Should be "pure", which means it does not mutate its arguments, perform side effects like API calls or modifying values outside of the function, or call non-pure functions like `Date.now()` or `Math.random()`.  This also means that updates should be done in an ***"immutable"*** fashion, which means **always returning new objects with the updated data**, rather than directly modifying the original state tree in-place.
+- Should be "pure", which means it does not mutate its arguments, perform side effects like API calls or modify values outside of the function, or call non-pure functions like `Date.now()` or `Math.random()`.  This also means that updates should be done in an ***"immutable"*** fashion, which means **always returning new objects with the updated data**, rather than directly modifying the original state tree in-place.
 
 >##### Note on immutability, side effects, and mutation
 > Mutation is discouraged because it generally breaks time-travel debugging, and React Redux's `connect` function: