x/pkgsite: fix badge image and pkg.go.dev link URLs in badge generation tool

[jamalc]

What did you do?

Generating badges using the badge generation tool (https://pkg.go.dev/badge).

For example, generating a badge for the URL:

https://pkg.go.dev/net/url?tab=overview

What did you expect to see?

It should produce the following markdown snippet:

[![PkgGoDev](https://pkg.go.dev/badge/net/url?tab=overview)](https://pkg.go.dev/net/url?tab=overview)

What did you see instead?

And instead produces this snippet:

[![PkgGoDev](https://pkg.go.dev/net/url?tab=overview)](https://pkg.go.dev/badge/net/url?tab=overview)

[gopherbot]

Change https://golang.org/cl/242641 mentions this issue: content,internal: fix markdown badge snippet

[gopherbot]

Change https://golang.org/cl/242737 mentions this issue: content/static: fix badge dynamic update