[skip ci] Add comment for msgQueryNodeIsUnhealthy (#12111)

Signed-off-by: bigsheeper <yihao.dai@zilliz.com>

internal/querynode/errors.go

@@ -16,7 +16,7 @@ import (
 	"fmt"
 )
 
-// error msg of unhealthy query node
+// msgQueryNodeIsUnhealthy is the error msg of unhealthy query node
 func msgQueryNodeIsUnhealthy(nodeID UniqueID) string {
 	return fmt.Sprintf("query node %d is not ready", nodeID)
 }