Add missing X in starter Square component (#5445)

This change adds missing "X" character for starter Square component showcase in "Overview" section

beta/src/content/learn/tutorial-tic-tac-toe.md

@@ -291,7 +291,7 @@ The `App.js` file should be selected in the _Files_ section. The contents of tha
 
 ```jsx
 export default function Square() {
-  return <button className="square"></button>;
+  return <button className="square">X</button>;
 }
 ```