Update keyboard shortcuts to meet the new GitHub content style guide

[ramyaparimi]

Code of Conduct

• I have read and agree to the GitHub Docs project's Code of Conduct

What article on docs.github.com is affected?

https://docs.github.com/en/desktop/installing-and-configuring-github-desktop/overview/keyboard-shortcuts

What part(s) of the article would you like to see updated?

Keyboard shortcuts in this document are outdated. Refer to Keyboard Shortcuts section in the content style guide on how to correctly present keyboard shortcuts in GitHub docs.

Additional information

commandk should be Command+K
commandshift7 should be Command+Shift+7