rustbook: Fix relative links on the Introduction page

rustbook: Fix relative links on the Introduction page

The Introduction page generated by rustbook used weird relative links
like "./getting-started.html" instead of just "getting-started.html"
like on the other pages. This adversely affected Windows builds the
worst, since it generated links like ".\getting-started.html" (note the
backslash). If you then try to upload the generated book to a webserver,
you end up with 404's. See this example of what is going on with the
Introduction page links and why this PR should fix it:
http://is.gd/fRUTXk

Compare the links on these two pages, for instance:
https://doc.rust-lang.org/nightly/book/
https://doc.rust-lang.org/nightly/book/getting-started.html

Also, fix a few whitespace issues in build.rs.