chore: add missing formatting directive in Go test (#3681)

diff --git a/kythe/go/platform/tools/kzip/mergecmd/mergecmd.go b/kythe/go/platform/tools/kzip/mergecmd/mergecmd.go
index 2773c87..8cbbcc6 100644
--- a/kythe/go/platform/tools/kzip/mergecmd/mergecmd.go
+++ b/kythe/go/platform/tools/kzip/mergecmd/mergecmd.go
@@ -58,7 +58,7 @@
 		return c.Fail("required --output path missing")
 	}
 	if err := mergeArchives(ctx, c.output, fs.Args()); err != nil {
-		return c.Fail("Error merging archives: ", err)
+		return c.Fail("Error merging archives: %v", err)
 	}
 	return subcommands.ExitSuccess
 }