x/pkgsite: anchor links to package-scoped constants and variables appear off-screen

[dmitshur]

Consider the following link to a package-scoped variable (but this applies to constants too):

https://pkg.go.dev/io#ErrShortWrite

I expect to be able to use it to see and point other people to the io.ErrShortWrite variable and its documentation. It largely works, but the ErrShortWrite variable shows up slightly off-screen, covered up by the sticky header:

It would be better if it weren't covered up, like so:

This minor problem doesn't affect godoc.org and golang.org/pkg:

/cc @julieqiu