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

add note on readme

<!-- Please describe your changes on the following line: -->
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]

---
<!-- 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
- [ ] These changes do not require tests because _____

<!-- 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. -->

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/19628)
<!-- Reviewable:end -->

README.md

@@ -230,9 +230,13 @@ cp servobuild.example .servobuild
 
 ## Running
 
-Use `./mach run [url]` to run Servo. Like so,
+Run Servo with the command:
 
-``` shell
+```sh
+./servo [url] [arguments] # if you run with nightly build
+./mach run [url] [arguments] # if you run with mach
+
+# For example
 ./mach run https://www.google.com
 ```