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.

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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.

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- [x] `./mach test-tidy` does not report any errors
- [ ] These changes fix #___ (GitHub issue number if applicable)

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- [ ] There are tests for these changes OR
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docs/HACKING_QUICKSTART.md

@@ -148,7 +148,7 @@ Before starting the debugger right away, you might want to get some information
 A typical command might be:
 
 ``` shell
-./mach run -d -- -i -y 1 --debug dump-layer-tree /tmp/a.html
+./mach run -d -- -i -y 1 --debug dump-style-tree /tmp/a.html
 ```
 
 … to avoid using too many threads and make things easier to understand.