fix Lua logo so that it can be read even without color

[yutkat]

I am calling onefetch from on Neovim and I am concerned that the Lua logo will be unreadable.
I modified it so that I could see it without the color.

before:

after:

[o2sh]

hmm, I've never considered this use case, we may have other logos like this.

Anyway, I took the opportunity to do some minor retouching on the logo to make it more symmetrical:

Thanks for your contribution @yutkat