Update git_metrics example to work properly in clean repo

[koiralakiran1]

Bug Report

Many people use the examples from the docs directly, as they are somewhat of a sound goto configuration after defaults, that doesn't require much tweak. Although it's just an example, I think the example of git_metrics has (I'm guessing) "unintentional" output for clean repos. If this was intentionally kept in the docs, we can close this issue.

Current Behavior

This is the current example in git_metrics module in the documentation

# ~/.config/starship.toml

[git_metrics]
added_style = 'bold blue'
format = '[+$added]($added_style)/[-$deleted]($deleted_style) '

Looks fine when the repo is dirty (+1/-1)

Shows +/- even when the repo is clean

Expected Behavior

There should be no residual output on a clean repo, with the example configuration from the docs.

Possible Solution

Update example in the docs to the following:

# ~/.config/starship.toml

[git_metrics]
added_style = 'bold blue'
# Note: the wrapping ( ) will result in empty string if all the variables within it are empty
format = '([+$added]($added_style)/[-$deleted]($deleted_style) )'