docs: fix links (#226)

Author: junagao

src/exercise/02.md

@@ -110,7 +110,7 @@ so it's useful to understand.
 
 > PLEASE NOTE: there was a subtle change in the order of cleanup functions
 > getting called in React 17:
-> https://github.com/kentcdodds/react-hooks/issues/90
+> <https://github.com/kentcdodds/react-hooks/issues/90>
 
 ## 🦉 Feedback
 

src/exercise/04.md

@@ -51,7 +51,7 @@ also be determined based on the value of `squares` and we've provided a
 📜 Read more about derived state in
 [Don't Sync State. Derive It!](https://kentcdodds.com/blog/dont-sync-state-derive-it)
 
-### Alternate:
+### Alternate
 
 If you'd prefer to practice refactoring a class that does this to a hook, then
 you can open `src/exercise/04-classes.js` and open that on
@@ -82,7 +82,7 @@ quick extra credit).
 
 [Production deploy](https://react-hooks.netlify.app/isolated/final/04.extra-3.js)
 
-Open `http://localhost:3000/isolated/final/04.extra-3.js` and see that the extra
+Open [http://localhost:3000/isolated/final/04.extra-3.js](http://localhost:3000/isolated/final/04.extra-3.js) and see that the extra
 version supports keeping a history of the game and allows you to go backward and
 forward in time. See if you can implement that!
 

src/exercise/05.md

@@ -59,7 +59,7 @@ Additionally, we'll need to clean up after ourselves if this component is
 unmounted. Otherwise we'll have event handlers dangling around on DOM nodes that
 are no longer in the document.
 
-### Alternate:
+### Alternate
 
 If you'd prefer to practice refactoring a class that does this to a hook, then
 you can open `src/exercise/05-classes.js` and open that on