Doc style: British spellings, e.g. "color" -> "colour"

[dansanderson]

The MEGA65 documentation style uses British spelling for words like "colour." Some documentation text is pulled from mega65-libc doc comments, and mega65-libc currently uses "color" in prose. To stay consistent with the declared style, we should change these instances to "colour" in the doc comments. Also look for other British spelling issues, e.g. "-ize" to "-ise". (See mega65-userguide's styleguide.pdf.)

Note that this rule does not apply to API names, only to prose descriptions. Commodore APIs use American spellings (COLOR). What mega65-libc should do for API names I don't know, but IMO it's fair for mega65-libc to make its own decision for API names. I'm only concerned with style for doc strings that get injected into the MEGA65 documentation.