Add "color space" to Commonly Misspelled Terms

[tabatkins]

In whatwg/html#6562 (comment) an HTML proposal had a back-and-forth about whether "color space" should be "colorspace" (as it would influence the spelling of a web-exposed attribute, either colorspace or colorSpace).

As referenced in that conversation, we already have another web-facing API that assumes it's "color space" and cases an attribute accordingly, so this latest proposal adopted that as well.

In reviewing CSS's usage (as our specs were cited in the discussion as well) I found that we were inconsistent, but in w3c/csswg-drafts#6217 @svgeesus determines that "color space" is indeed the industry-standard spelling, which is luckily consistent with the above-mentioned APIs, and fixes our Color specs to consistently use that term as well.

Since this is a common mistake, it should be listed in the style guide as well.