(docs) add link to freeCodeCamp

Author: jcubic

docs/docs/deep-dive/echo.mdx

@@ -127,9 +127,10 @@ const term = $('body').terminal({}, {
 fonts, on [figlet demo website](https://patorjk.com/software/taag/) (type some text and click test
 all to see all fonts).
 
-`$.terminal.figlet` return a function that renders the text using figlet, because of this you have
-responsive text. See below demo how it works, you can resize the window, to see how the ASCII Art
-change.
+`$.terminal.figlet` [returns a
+function](https://www.freecodecamp.org/news/higher-order-functions-explained/) that renders the text
+using figlet, because of this you have responsive text. See below demo how it works, you can resize
+the window, to see how the ASCII Art change.
 
 <CodePen id="xbKqpZG" title="Terminal Figlet" />