Fixing typos/method calls in grabValueFrom

grabValueFrom confusing comments and incorrect call to non-existent attr
function on DOMElement.
- filter() returns Crawler
- Crawler does not have a childNodes property
- children() returns DOMElement objects

I'm hoping this is small+obvious enough to not require a test case/Issue # :).  If it does let me know and I'll see about adding to this when I get some more time.