Auto merge of #11856 - nerith:doc, r=jdm

README.md: Update keyboard shortcuts

Backspace and Shift-Backspace are no longer used to go backwards and forwards through history.

Instead, Alt+Left Arrow and Alt+Right Arrow are used now.

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README.md

@@ -181,8 +181,8 @@ URL with servo).
 
 - `Ctrl--` zooms out
 - `Ctrl-=` zooms in
-- `Backspace` goes backwards in the history
+- `Alt`+`left arrow` goes backwards in the history
-- `Shift-Backspace` goes forwards in the history
+- `Alt`+`right arrow` goes forwards in the history
 - `Esc` exits servo
 
 ## Developing