testing: fix minor bug with mutex profile error message
testing: fix minor bug with mutex profile error message

Change-Id: I92b7a44476cfd9e2f099477b9d7903bbaa2fd8da
Reviewed-on: https://go-review.googlesource.com/c/go/+/173720
Run-TryBot: Caleb Spare <dce7183dd721072d236fed9264529050987c94bf@gmail.com>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

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