Fix wrong comment (-> wrong Doxygenation) about the meaning of

Fix wrong comment (-> wrong Doxygenation) about the meaning of
SetWriteModeToRaw() :
    Tells the writer we want to keep 'Grey pixels + Palettes color'
    when possible (as opposed to convert 'Palettes color' to RGB)
 NOT
    Tells the writer we want to write only the pixels (no header)