neovim: Improve icon

[why]
The left leg is far too thin.
The gaps between the left legs are rather small.
That makes noticing the details hard, but that are decisive elements.

[how]
Manually edit in Inkscape...

Reported-by: berlinx2104
Signed-off-by: Fini Jastrow <ulf.fini.jastrow@desy.de>