x/pkgsite: Examples on golang.org/x/exp/xerrors are broken

[3456091]

What is the URL of the page with the issue?

https://pkg.go.dev/golang.org/x/exp/xerrors

What is your user agent?

Firefox Nightly

What did you do?

Tried to run the example for, say, the As method.

What did you expect to see?

The result of the example.

What did you see instead?

go: finding module for package golang.org/x/exp/xerrors
go: downloading golang.org/x/exp v0.0.0-20220328175248-053ad81199eb
go: finding module for package golang.org/x/exp/xerrors
prog.go:7:2: module golang.org/x/exp@latest found (v0.0.0-20220328175248-053ad81199eb), but does not contain package golang.org/x/exp/xerrors

[jamalc]

Duplicate of #50769