cas: fix flag name in error

Change-Id: Ic83b5ccdff70e2da7b0a9d2da642813b982376da
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2902171
Auto-Submit: Takuto Ikuta <tikuta@chromium.org>
Commit-Queue: Yoshisato Yanagisawa <yyanagisawa@google.com>
Reviewed-by: Yoshisato Yanagisawa <yyanagisawa@google.com>
Reviewed-by: Junji Watanabe <jwata@google.com>
diff --git a/client/cmd/cas/lib/download.go b/client/cmd/cas/lib/download.go
index a81fc3c..40b2558 100644
--- a/client/cmd/cas/lib/download.go
+++ b/client/cmd/cas/lib/download.go
@@ -95,7 +95,7 @@
 	}
 
 	if r.kvs != "" && r.cacheDir == "" {
-		return errors.New("if small-files-cache is set, cache-dir should be set")
+		return errors.New("if kvs-dir is set, cache-dir should be set")
 	}
 
 	r.dir = filepath.Clean(r.dir)