README.md: clarify buildtools versions are fixed

[dhalperi]

When I read "current time" I didn't know whether it meant:

• release time
• bazel sync time
• command runtime

Digging in, the version is hardcoded: https://github.com/keith/buildifier-prebuilt/blob/fa9175bbbcbfb5a782d262af73c2cb36b9cab99b/defs.bzl#L39-L41

https://github.com/keith/buildifier-prebuilt/blob/0b8177deda07ab95f69539c5536623b0678a169a/buildtools.bzl#L146-L161

I think this small language tweak helps.

[keith]

thanks!