Fix GitHub link unconditionally navigating to Discord (#1483)

The GitHub link in the site header across all pages is unconditionally
linking to Discord, not GitHub. More details in the issue, but the gist
is it appears to be a regression from a large refactor merged 2 days
ago: fef6139dc457914a8aa67eae81f5be6dfc300e1c

All the context was already wired up, it just appears to be using the
wrong context variable. An easy fix! Hopefully.

Tested locally by smoke testing the link across a few pages and project
roots.

Closes https://github.com/solidjs/solid-docs/issues/1482