fix(challenges): Change of text in Introduction to the basic node a

Text was not getting changed going from step 1 to step 2 because ul
cannot be child node of p tag so I changed p tag to div in Introduction
to the basic node and express challenges. For ref.
https://stackoverflow.com/questions/10601345/ul-element-can-never-be-a-child-of-p-element

BREAKING CHANGE: Changed p tag to div tag in Step.jsx

Closes #15796

common/app/routes/challenges/views/step/Step.jsx

@@ -204,7 +204,7 @@ export class StepChallenge extends PureComponent {
             smOffset={ 1 }
             xs={ 12 }
             >
-            <p
+            <div
               className={ `${ns}-description` }
               dangerouslySetInnerHTML={{ __html: info }}
             />