fix some instructions

exercises/02.side-effects/01.problem.effects/README.mdx

@@ -9,7 +9,7 @@ the URL search params without triggering a full-page refresh. So whenever the
 user clicks "submit," we update the URL search params. The trouble is, when they
 hit the back button, the search doesn't stay synchronoized with the URL (yet).
 
-You can take a look at <DiffLink app1={-1}>my changes</DiffLink> for
+You can take a look at <DiffLink app1={-1} app2={0}>my changes</DiffLink> for
 details on what I did.
 
 👨‍💼 Thanks Kellie. So what we you to do is make it so when the user hits the back

exercises/03.lifting-state/01.problem.lift/README.mdx

@@ -1,7 +1,7 @@
 # Lift State
 
 🧝‍♂️ I refactored the app a bit. In the process I kinda broke stuff and you'll be
-required to fix it. Feel free to <DiffLink app1={-1}>check the diff</DiffLink>
+required to fix it. Feel free to <DiffLink app1={-1} app2={0}>check the diff</DiffLink>
 to know what I've done. I pretty much just moved stuff into separate components
 to get a little more organization in here.
 

exercises/04.dom/01.problem.ref/index.tsx

@@ -7,7 +7,7 @@ interface HTMLVanillaTiltElement extends HTMLDivElement {
 }
 
 function Tilt({ children }: { children: React.ReactNode }) {
-	// 🐨 create a tiltRef here with useRef<HTMLVanillaTiltElement>()
+	// 🐨 create a tiltRef here with useRef<HTMLVanillaTiltElement>(null)
 
 	useEffect(() => {
 		// 🐨 get the tiltNode from tiltRef.current

exercises/05.unique-ids/01.problem.use-id/README.mdx

@@ -12,7 +12,7 @@ they do pass an `id` we should use the `id` they passed instead of a generated
 one (maybe they want a very specific `id` for some reason and we should support
 both!).
 
-As usual, you can <DiffLink app1={-1}>check my work</DiffLink> if you like.
+As usual, you can <DiffLink app1={-1} app2={0}>check my work</DiffLink> if you like.
 
 👨‍💼 So your job is to fix the label association. You'll notice that the checkbox
 is working as expected already because it's not using the `Field` component due

exercises/06.tic-tac-toe/03.problem.history/README.mdx

@@ -7,6 +7,6 @@ allows users to go backward through the game and change moves.
 As usual, you can check <Link to=".?preview=solution">the solution</Link> to
 check what the final experience should be like.
 
-🧝‍♂️ I made a couple changes you can <DiffLink app1={-1}>check here</DiffLink> if
+🧝‍♂️ I made a couple changes you can <DiffLink app1={-1} app2={0}>check here</DiffLink> if
 you'd like. I mostly just refactored things a bit to get it ready for your work.
 Should be a fun feature to add! Enjoy!