runtime/pprof: update comments to point to new pprof

In the comments for this file there is a reference to gperftools
for more info on pprof. pprof now live on its own repo on github,
and the version in gperftools is deprecated.

Change-Id: I8a188f129534f73edd132ef4e5a2d566e69df7e9
Reviewed-on: https://go-review.googlesource.com/24502Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/pprof/pprof.go

@@ -5,7 +5,7 @@
 // Package pprof writes runtime profiling data in the format expected
 // by the pprof visualization tool.
 // For more information about pprof, see
-// http://code.google.com/p/google-perftools/.
+// http://github.com/google/pprof/.
 package pprof
 
 import (