x/website: broken formatting for the "Download Go" element in install page

[ALTree]

Visit https://tip.golang.org/doc/install

How the first section used to look:

how it looks now:

The

Download Go Click here to visit the downloads page

part looks like broken html/css.

[ALTree]

cc @andybons

[gopherbot]

Change https://golang.org/cl/195519 mentions this issue: x/website: Add css to style doc/install download button