Change recommended vim keybinding for "reset file"

[callpraths]

This Pull Request fixes/closes #2704

Avoids a hindenbug due to two interpretations of Shift+U

• It is the default key-binding to "undo last commit"
• It is the key binding for "reset file" in the recommended vim-style bindings

There is a potential for confusion between those two actions and they have qualitatively different outcomes - the default binding is more of a user-data destroying action (though there's always reflog...)

This PR updates the suggested vim binding to lowercase u. This makes u the keybinding for:

• "reset file" in status view.
• "reset current hunk" in diff view.

Those two actions have the same scope - undo unstaged changes in working directory, so having the same key-binding makes sense.

I followed the checklist:

• I added unittests [N/A: no functional changes]
• I ran make check without errors
• I tested the overall application
• I added an appropriate item to the changelog