Update module-factories.md

[dear-lizhihua]

[skipjack]

@dear-lizhihua that page actually does belong in the "Plugins" section though I see how it can be confusing. Note that I do have #1754 open that refactors and standardizes the whole plugins section which should make things more clear. I took out the pages that mostly contained TODOs but will re-add some of those pages like this one later on (as the factories are tapable).

I'll make sure that it's clear why this page is in the "Plugins" section and maybe rename it to "Factories" to prevent confusion. I'm hoping to make the final push on that branch this week.