Typo in img2sixel man about -d option

[tsutsui]

man img2sixel shows about -d option as the following:

       -d DIFFUSIONTYPE, --diffusion=DIFFUSIONTYPE
            choose diffusion method which used with color reduction.
            DIFFUSIONTYPE is one of them:
            auto     -> choose diffusion type automatically (default)
            none     -> do not diffuse
            fs       -> Floyd-Steinberg method
            atkinson -> Bill Atkinson's method
            jajuni   -> Jarvis, Judice & Ninke
            stucki   -> Stucki's method
            burkes   -> Burkes' method
            a_dither -> positionally stable arithmetic dither
            a_dither -> positionally stable arithmetic xor based dither

It looks the last one should be x_dither, not a_dither, as img2sixel -H says. https://github.com/saitoha/libsixel/blob/5db717dfef6fa327cd4025e7352550f63d20699c/converters/img2sixel.1#L99 https://github.com/saitoha/libsixel/blob/5db717dfef6fa327cd4025e7352550f63d20699c/converters/img2sixel.c#L173

[saitoha]

@tsutsui Thanks!