Fix "preparing the build" chapter linking back to a previous chapter as next step

Created by: Nickforall

As pointed out in #122 (closed), the chapter "preparing the build" says the next step is to install the toolchain, which is the previous chapter, while "install the toolchain" is skipping "preparing the build" and going straight to "compiling Redox".

This can be confusing for readers, so I changed the links in the "next step" paragraph to follow the order of the chapters.