docs: fb: Add font_6x8 to available built-in fonts

Recently we added a new 6x8 font in commit e2028c8e ("lib/fonts: add
font 6x8 for OLED display"). Add its name to the "compiled-in fonts"
list.

Signed-off-by: Peilin Ye <yepeilin.cs@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20201018205401.698242-1-yepeilin.cs@gmail.com