testing: fix testing flags link
testing: fix testing flags link

Fixed a broken link to a section in the documentation for the
test flags for the go command.

Change-Id: Ic4bdd4965aac7856dd13a2adda9d774b9bae4113
GitHub-Last-Rev: 15bda340674c268a92f43a2820eca50482940de1
GitHub-Pull-Request: golang/go#24613
Reviewed-on: https://go-review.googlesource.com/103835
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

josharian/go,golang/go,cloudflare/go,CAFxX/go,CAFxX/go,josharian/go,josharian/go,Cofyc/go,CAFxX/go,Cofyc/go,CAFxX/go,josharian/go,golang/go,nawawi/go,nawawi/go,josharian/go,nawawi/go,akutz/go,golang/go,josharian/go,akutz/go,cloudflare/go,golang/go,josharian/go,Cofyc/go,CAFxX/go,nawawi/go,Cofyc/go,cloudflare/go,cloudflare/go,Cofyc/go,golang/go,akutz/go,akutz/go,akutz/go,cloudflare/go,Cofyc/go,golang/go,akutz/go,CAFxX/go,akutz/go,CAFxX/go,Cofyc/go,golang/go,cloudflare/go,josharian/go,akutz/go,nawawi/go,nawawi/go,cloudflare/go,nawawi/go,Cofyc/go,golang/go,nawawi/go,CAFxX/go,cloudflare/go