Fix keybindings title

[Pranomvignesh]

This PR fixes issue in monaco-editor#2468

Description :

• I have changed the value of altKey in AriaLabelProvider in the file keybindingLabels.ts

Testing :

• I hovered over the shortcut binding inside the quick input command palette and that is now properly showing as Option

[Pranomvignesh]

@alexdima Are Contributors allowed to refactor the test files? As I can see that most of the assertions failed because it is expecting an improper value

For example :
One test case is expecting Alt+A for MacOS instead of Opt+A in keybindingIO.test.ts file

Correct me if I am wrong

[alexdima]

Thank you!