doc: update info about flame graphs in diagnostics.html
doc: update info about flame graphs in diagnostics.html

Use Brendan Gregg's FlameGraphs page link.
Mention the flame graph is available from the upstream pprof.

Change-Id: Ife1d5a5f4f93f20cd5952a09083f798b77d25a60
Reviewed-on: https://go-review.googlesource.com/83798
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

codestation/go,Cofyc/go,CAFxX/go,golang/go,cloudflare/go,Cofyc/go,nawawi/go,CAFxX/go,golang/go,nawawi/go,akutz/go,nawawi/go,CAFxX/go,nawawi/go,cloudflare/go,nawawi/go,akutz/go,CAFxX/go,christopher-henderson/Go,josharian/go,CAFxX/go,codestation/go,josharian/go,josharian/go,cloudflare/go,josharian/go,golang/go,Cofyc/go,cloudflare/go,golang/go,Cofyc/go,nawawi/go,codestation/go,akutz/go,cloudflare/go,codestation/go,josharian/go,akutz/go,cloudflare/go,josharian/go,akutz/go,akutz/go,cloudflare/go,codestation/go,akutz/go,golang/go,golang/go,nawawi/go,Cofyc/go,josharian/go,golang/go,codestation/go,akutz/go,Cofyc/go,josharian/go,codestation/go,cloudflare/go,CAFxX/go,Cofyc/go,CAFxX/go,nawawi/go,codestation/go,CAFxX/go,Cofyc/go,golang/go