doc: readme: make logo text gray

The logo did not display black text when using the Github light theme in
a system configured to prefer dark mode. An alternative solution: just
make text grayish (#4e4e4e) so that it looks "ok" in both light and dark
modes.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>
1 file changed