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

Documentation/fb/fbcon.rst

@@ -81,7 +81,7 @@ C. Boot options
 1. fbcon=font:<name>
 
 	Select the initial font to use. The value 'name' can be any of the
-	compiled-in fonts: 10x18, 6x10, 7x14, Acorn8x8, MINI4x6,
+	compiled-in fonts: 10x18, 6x10, 6x8, 7x14, Acorn8x8, MINI4x6,
 	PEARL8x8, ProFont6x11, SUN12x22, SUN8x16, TER16x32, VGA8x16, VGA8x8.
 
 	Note, not all drivers can handle font with widths not divisible by 8,