missing initial_utm_parameters from default properties doc

[caracheng]

In this document, we mention that:

Mixpanel's Javascript library will also track initial_utm_parameters as a profile property, based on the first ever visit. This is helpful as if a user makes a purchase or completes some other important event, it is important to know want to know what acquisition channel brought them to your site originally.

However, in the default properties doc for Web, we don't include the initial_ utm_parameters that we track by default in the table.

We should update the table to reflect that we do track this info by default as profile properties

[ranic]

@caracheng -- do you mind making a PR to add this?

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