Use boolean instead of bool in LabeledElement docs

When James tried to update the version of OOjs UI used in VE,
this happened, preventing it from being merged:
Warning: /path/to/oojs-ui/oojs-ui.js:2451: Unknown type bool

Change-Id: I9b1619ddc7f3cebb56f8c78f3176e2e43951a186