Make the GitBook icon easier to see

[gewaleelek]

It's almost invisible in GitHub dark mode 😅

[gewaleelek]

I tried to make the new GitBook button in the style of other buttons in the readme:

The GitBook icon comes from the official site, but I did change the stroke color and width. I'm not sure if that is prohibited by whatever GitBook license we are under.

Any thoughts/suggestions?

[Donnnno]

Sorry, finally updated it!