static/frontend/about: don't use relative link for issue
Fixes
Trust: Ian Lance Taylor
Run-TryBot: Ian Lance Taylor
TryBot-Result: kokoro
<p>
You can add links to your README files and package documentation that will be
shown on the right side of the pkg.go.dev page.
- For details, see <a href="/issue/42968">this issue</a>.
+ For details, see <a href="https://go.dev/issue/42968">this issue</a>.
</p>
<h2 id="keyboard-shortcuts">Keyboard Shortcuts</h2>
