It is such a small intervention that I wouldn't really consider it a breach of the process, It's like making an "observation about grammar" while a student talks, if it was about content or logic than I would agree with you, but stepping in here doesn't alter in any way the thought process of the proposer or modify the meaning of what's written. I would minimze the issue and tell the proposer right away, "let's add an -ing to your starting verb, this is just an holacracy syntax convention to better convey the feeling of something that is ongoing ". If you don't do anything the pop-up will raise questions in the mind of the proposer, that might interrupt his thought process even more than stepping in to explain. In a separate coaching phase you could then clearly explain while it is important that accountabilities have this "ongoing feeling" to them, i.e. they clearly distinguish them from outputs which belong in other artifacts and phases of holacracy practice (projects and actions).
ADD ON: I might have taken Jean Michel's approach if the popup wasn't there because in that case I (not the tool) would have interrupted the thought process of the proposer.