Fix HTML and JavaScript introduction links (#7339)
Fix HTML and JavaScript introduction links (#7339)

The HTML link pointed to the JavaScript introduction and the JavaScript link pointed to the HTML introduction. Correct the URLs for each link.
0x80/gatsby,gatsbyjs/gatsby,mingaldrichgan/gatsby,0x80/gatsby,gatsbyjs/gatsby,ChristopherBiscardi/gatsby,mingaldrichgan/gatsby,gatsbyjs/gatsby,mingaldrichgan/gatsby,ChristopherBiscardi/gatsby,gatsbyjs/gatsby,ChristopherBiscardi/gatsby,0x80/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby,ChristopherBiscardi/gatsby