Update the world index page markdup from h3 to h2

[bhumi1102]

This PR is to address this SEO task https://3.basecamp.com/4529592/buckets/41325723/todos/8488924674

[bhumi1102]

@jathayde Hi John ! Another SEO task with the an heading change but the desire to keep the design/CSS unchanged. I got it working using a similar technique as previously - adding an id: events on the page and then targeting that page in the .scss file. This definitely feels hacky but let me know if you see a better class to target, I'm using common-content.

[jathayde]

This looks the same to me for desktop and mobile, which is what we want.

I do like your pattern of namespacing page changes. I do that often on sites I build, so I agree with that pattern.

My only question (and it's more one of how do we want to do this going forward as opposed to saying you should change it) - should the H2 override for events be immediately after H2, or should we have a section for Hn overrides (we only have one so far, but basically what you've got set up right now). This would apply to both the primary and media-query areas.