Fix comma position (#4897)

Fix comma position at line 219 , in phrase When you need to "change things,"

beta/src/pages/learn/keeping-components-pure.md

@@ -216,7 +216,7 @@ Every new React feature we're building takes advantage of purity. From data fetc
   * **Same inputs, same output.** Given the same inputs, a component should always return the same JSX. 
 * Rendering can happen at any time, so components should not depend on each others' rendering sequence.
 * You should not mutate any of the inputs that your components use for rendering. That includes props, state, and context. To update the screen, ["set" state](reacting-to-input-with-state) instead of mutating preexisting objects.
-* Strive to express your component's logic in the JSX you return. When you need to "change things," you'll usually want to do it in an event handler. As a last resort, you can `useEffect`.
+* Strive to express your component's logic in the JSX you return. When you need to "change things", you'll usually want to do it in an event handler. As a last resort, you can `useEffect`.
 * Writing pure functions takes a bit of practice, but it unlocks the power of React's paradigm.
 
 </Recap>
@@ -857,4 +857,4 @@ It is useful to remember which operations on arrays mutate them, and which don't
 
 </Solution>
 
-</Challenges>
+</Challenges>