runtime: fix the absolute URL to pprof tools

Change-Id: I82eaf5c14a5b8b9ec088409f946adf7b5fd5dbe3
Reviewed-on: https://go-review.googlesource.com/27311Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/cpuprof.go

@@ -4,7 +4,7 @@
 
 // CPU profiling.
 // Based on algorithms and data structures used in
-// http://code.google.com/p/google-perftools/.
+// https://github.com/google/pprof.
 //
 // The main difference between this code and the google-perftools
 // code is that this code is written to allow copying the profile data