If it's submitted, it's too late to fix on the primary. I might consider writing up a description anyway though. You'll be able to add 'additional information' on the secondaries, and you can include the description of the activity. If not, I'd bring it along to interviews. They always asked me if there was anything I wanted to add, and you could give them the description of the activity there.
But, as you did address it in the PS, it's probably not the end of the world. The description is available to adcoms, just not where it's supposed to be.