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/119395Reviewed-by: Ralph Corderoy <ralph@inputplus.co.uk>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/cover/cover_test.go

@@ -325,7 +325,7 @@ func TestCoverHTML(t *testing.T) {
 		}
 	}
 	if len(goldenLines) != len(outLines) {
-		t.Fatalf("output longer than golden; first extra output line %d: %q\n", len(goldenLines), outLines[len(goldenLines)])
+		t.Fatalf("output longer than golden; first extra output line %d: %q\n", len(goldenLines)+1, outLines[len(goldenLines)])
 	}
 }