x/pkgsite: "share" yields html in output pane

[srenatus]

What is the URL of the page with the issue?

Any page with examples, but this one happened on https://pkg.go.dev/testing

What is your user agent?

Irrelevant, but it's [Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/98.0.4758.109 Safari/537.36](https://developers.whatismybrowser.com/useragents/parse/?analyse-my-user-agent=yes)

Screenshot

What did you do?

I've clicked the "Share" button below the example.

What did you expect to see?

Something else. Just the URL maybe?

What did you see instead?

The html link. And a new tab opened, with the proper page opened.

[gopherbot]

Change https://go.dev/cl/392955 mentions this issue: static: fix playground share link ouput