Auto merge of #26085 - utsavoza:ugo/docs/01-04-2020, r=jdm

 Use a valid debug option in the example command in the Debugging guide.

<!-- Please describe your changes on the following line: -->
The debug option `dump-layer-tree` used seems maybe irrelevant to the example command, but using one from the existing debug options helps to simply copy/paste the command and see what the example is trying to demonstrate.

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `___` with appropriate data: -->
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [ ] These changes fix #___ (GitHub issue number if applicable)

<!-- Either: -->
- [ ] There are tests for these changes OR
- [x] These changes do not require tests because they are doc changes exclusively.

<!-- Also, please make sure that "Allow edits from maintainers" checkbox is checked, so that we can help you if you get stuck somewhere along the way.-->

<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->

0 files changed, 0 insertions, 0 deletions