servo: Merge #13511 - README.md: Make keyboard shortcut syntax similar (from nerith:doc); r=jdm

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<!-- 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

- [X] These changes do not require tests because it only changes documentation

Source-Repo: https://github.com/servo/servo
Source-Revision: d9f53cb14df4467de714e61f9a5ab4b4a510f5f5

servo/README.md

@@ -198,8 +198,8 @@ URL with servo).
 
 ### Keyboard Shortcuts
 
-- `Ctrl--` zooms out
-- `Ctrl-=` zooms in
+- `Ctrl`+`-` zooms out
+- `Ctrl`+`=` zooms in
 - `Alt`+`left arrow` goes backwards in the history
 - `Alt`+`right arrow` goes forwards in the history
 - `Esc` exits servo