Update legacy-mtu-billing.md (#1843)

Co-authored-by: mherrman <mikey.herrman@mixpanel.com>

Author: caracheng

pages/docs/pricing/legacy-mtu-billing.md

@@ -34,6 +34,7 @@ Certain events and API updates are non-qualifying and excluded from the MTU calc
 - Message Suppressed (`$message_suppressed`)
 - Message Unsubscribed (`$unsubscribe`)
 - Campaign Entered (`$journey_entered`)
+- Experiment Started (`$experiment_started`)
 - Mailchimp Campaign - Open
 - Mailchimp Campaign - Click
 - Mailchimp Campaign - Sent
@@ -93,4 +94,4 @@ The main scenarios that usually increase the number of MTUs:
 - A marketing campaign (can lead to an increase in tracked users due to an influx of anonymous visitors).
 - An update that adds tracking to new parts of your product.
 
-To assist in analyzing potential sources of MTU spikes, create a copy of [this board](https://mixpanel.com/project/2195193/view/139237/app/boards#id=5762529) from our demo project into the main project contributing to MTU charges. Once you open the board linked above, you can click on the 3 dots menu located on the top right-hand corner to duplicate the board, then move the duplicated board into your project.  
+To assist in analyzing potential sources of MTU spikes, create a copy of [this board](https://mixpanel.com/project/2195193/view/139237/app/boards#id=5762529) from our demo project into the main project contributing to MTU charges. Once you open the board linked above, you can click on the 3 dots menu located on the top right-hand corner to duplicate the board, then move the duplicated board into your project.