Auto merge of #19628 - tigercosmos:x3, r=KiChjang

add note on readme

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Add note about how to use command while running servo
origin  `readme` is not clear.
I think ./mach run [url] [arguments] is more clear than ./mach run [url]

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- [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).

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