Fix typo in reactive DOM description in Eliom 4

Hi,

I was taking a look to new implementation of the reactive DOM (introduced with eliom 4) and I found a little typo in the given code. I'm not sure if it was an intended typo or not (I've seen some issues about the naming of `AllChildren`/`Children`, maybe it is the same here for `appendChilds`/`appendChildren`), but anyway it is a little thing to change, but it could be confusing for fresh beginners on ocsigen. :)

I prefer to use a pull request here instead of fixing it directly on the repo since I'm not sure if it is due to an unintended typo.