Fix typo on learn/te-foundations/tailwind-css/flexbox/ (#2117)

change bg-reg-500 to bg-red-500

Signed-off-by: Michael <[email protected]>

Author: Mike2020x

site/content/learn/te-foundations/2-tailwindcss/20-flexbox/index.html

@@ -228,7 +228,7 @@ <h2>And what an exciting adventure it is!</h2>
   <h2 class="mb-6 mt-12 text-xl font-semibold">Step 3 - set a height</h2>
 
   <p>
-    Let's do an experiment - let's add the <code>bg-reg-500</code> class to the
+    Let's do an experiment - let's add the <code>bg-red-500</code> class to the
     <code>div</code> with our flexbox, which will give it a red background.
     Thanks to this, we will be able to see its actual height.
   </p>