runtime/pprof: update stale link in documentation

Fixes #25477

Change-Id: I6f724bb855cfffa21de090c1fcb04d58d7cdd9d8
Reviewed-on: https://go-review.googlesource.com/113839Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/pprof/pprof.go

@@ -68,7 +68,7 @@
 // all pprof commands.
 //
 // For more information about pprof, see
-// https://github.com/google/pprof/blob/master/doc/pprof.md.
+// https://github.com/google/pprof/blob/master/doc/README.md.
 package pprof
 
 import (