make backslashes visible in docs for dict_add and dict_set

[dws]

In order to escape colons in the keys and values that you provide to dict_add and dict_set, you must precede them with backslashes. The raw README.md file has the backslashes, but they are not visible when rendered. We here double these backslashes so that they'll be visible when rendered.

This is a followup to https://github.com/bazelbuild/buildtools/pull/1361.

[dws]

@oreflow FYI

[oreflow]

Nice, thanks for fixing this :)