Fix simple typo: childeren -> children

# Issue Type

[x] Bug (Typo)

# Steps to Replicate

1. Examine qtfaststart/processor.py.
2. Search for `childeren`.

# Expected Behaviour

1. Should read `children`.

Semi-automated issue generated by
https://github.com/timgates42/meticulous/blob/master/docs/NOTE.md

To avoid wasting CI processing resources a branch with the fix has been
prepared but a pull request has not yet been created. A pull request fixing
the issue can be prepared from the link below, feel free to create it or
request @timgates42 create the PR. Alternatively if the fix is undesired please
close the issue with a small comment about the reasoning.

https://github.com/timgates42/qtfaststart/pull/new/bugfix_typo_children

Thanks.