Fix documentation black/white reversal in Singles

[kurahaupo]

Remove "black" and "white" because they're inverted when using a dark/nighttime theme

[chrisboyle]

Thanks for this; unfortunately I cannot merge it in its current form:

• The HTML files are generated from puzzles.but using halibut, and the only reason they're checked in is so that developers working on anything other than docs don't need to install halibut. I keep meaning to tweak the generation process to insert a big warning comment at the top of the HTML explaining this, but haven't yet, sorry.
• While the change for Singles is small enough, this should probably be addressed coherently for all 40 puzzles at once, and many of the others are harder to describe in a colour-agnostic way.
• I don't want to make that level of change to the docs source as an Android-specific change, because it will impede all future merges from upstream that touch the docs.

There is light at the end of the tunnel though: Simon has mentioned wanting to add colour preferences upstream, at which point I imagine he would do something to make the docs more colour-agnostic, and then any smaller Android tweaks can be made starting from that point.