Changed unclickable link in The rustc book

[atlasgorn]

link https://github.com/Kobzol/cargo-pgo unclickable here

[rustbot]

r? @oli-obk

rustbot has assigned @oli-obk.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

[jieyouxu]

Can you please squash the commits?

[jieyouxu]

@rustbot author

[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

[JohnCSimon]

@atlasgorn
Ping from triage: please address comments from the reviewer. Thanks.

FYI: when a PR is ready for review, send a message containing
@rustbot ready to switch to S-waiting-on-review so the PR is in the reviewer's backlog.