Typo: "af a user" should be "if a user" in Chapter 2

[jonathan-barrios]

Where?

Chapter 2: HTTP. Express server. Next-Express server, nodemon...
Section: MenuWithAvatar and Header components

Currently

"For example, af a user is logged out, show that user a link to the Login page; if a user is logged in, show that user's avatar with a dropdown menu."

Corrected

"For example, if a user is logged out, show that user a link to the Login page; if a user is logged in, show that user's avatar with a dropdown menu."

I'm happy to submit a PR if you'd prefer that approach.