cmd/cover: fix off-by-one in test error message
cmd/cover: fix off-by-one in test error message

Drive-by after previous CL.

Change-Id: I87db65b65745a0d76500cce06ac276b0d7928404
Reviewed-on: https://go-review.googlesource.com/119395
Reviewed-by: Ralph Corderoy <d6424ecc823ca663bdcbaea8e2d9df58a4d0e2b8@inputplus.co.uk>
Reviewed-by: Daniel Martí <29bf28b42a938ace12a5819da4ed4ba03d82f315@mvdan.cc>
Run-TryBot: Daniel Martí <29bf28b42a938ace12a5819da4ed4ba03d82f315@mvdan.cc>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

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