runtime/pprof: update outdated google/pprof link
Google/pprof changed the master branch to main, so it might be better to update it to the latest.
Change-Id: Id29aca80a9a83a9c10da215180ad65816bc88936
GitHub-Last-Rev:
0023c28dc0d84a2529e24c694acfed7a363d2d0b
GitHub-Pull-Request: golang/go#65792
Reviewed-on: https://go-review.googlesource.com/c/go/+/565177
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>