rustdoc: display problem for tooltip on error index page

[jsha]

Steps to reproduce (Chrome on Linux):

1. Visit https://doc.rust-lang.org/nightly/error_codes/E0283.html
2. Hover the (i) icon.

Results:

There is a little spur of black hanging below the tooltip.

cc @GuillaumeGomez

[GuillaumeGomez]

Thanks! I'll take a look tomorrow.