Documentation improvement regarding markup directive

[MangelMaxime]

Hello,

Documentation says:

i3bar supports Pango markup, allowing your format strings to specify font, color, size, etc. by setting the markup directive to "pango".

It took me a while to understand that the way to set up markup was to do

general {
    colors = true
    interval = 5
    markup = "pango"
}

I think it could make sense to add markup = "pango" in the Sample configuration like it seems to be done for others settings.

I can send a PR, if agreed