fix: Update mappings README

[zenbro]

First of all, thank you for such a great and clean plugin!

I noticed that all the mapping examples in the "GutterMarks Actions" section within the README contain an error: vim.keymap.set expects the mode ("n", "v", etc.) as the first argument. If these mappings are copied/pasted into the config, it will cause errors on nvim startup.
I've fixed this.

[NickvanDyke]

May be worth adding { desc = ... } as well 😀 optionally replacing the preceding comments

[zenbro]

Sure! { desc = ... } will be very handy. I've updated the commit and removed the preceding comments.

[dimtion]

Thank you for the fix, indeed I use lazy.nvim so I did not check that the example mappings in the README were without typos in native nvim.