x/website: CSS breakpoint in nav is wrong

[julieqiu]

Only the bottom half of the Go logo is visible when resizing the browser, somewhere between Desktop and mobile sizes.

/cc @andybons

[gopherbot]

Change https://golang.org/cl/190408 mentions this issue: content/static: apply various stylistic fixes

[andybons]

Fix does not take into account scrollbars

[gopherbot]

Change https://golang.org/cl/190413 mentions this issue: content/status: account for vertical scrollbar in nav breakpoint