fix flag name in query frontend doc (#4843)

Signed-off-by: Ben Ye <[email protected]>

docs/components/query-frontend.md

@@ -105,7 +105,7 @@ Naming is hard :) Please check [here](https://github.com/thanos-io/thanos/pull/2
 
 ## Recommended Downstream Tripper Configuration
 
-You can configure the parameters of the HTTP client that `query-frontend` uses for the downstream URL with parameters `--query-range.downstream-tripper-config` and `--query-range.downstream-tripper-config-file`. If it is pointing to a single host, most likely a load-balancer, then it is highly recommended to increase `max_idle_conns_per_host` via these parameters to at least 100 because otherwise `query-frontend` will not be able to leverage HTTP keep-alive connections, and the latency will be 10 - 20% higher. By default, the Go HTTP client will only keep two idle connections per each host.
+You can configure the parameters of the HTTP client that `query-frontend` uses for the downstream URL with parameters `--query-frontend.downstream-tripper-config` and `--query-frontend.downstream-tripper-config-file`. If it is pointing to a single host, most likely a load-balancer, then it is highly recommended to increase `max_idle_conns_per_host` via these parameters to at least 100 because otherwise `query-frontend` will not be able to leverage HTTP keep-alive connections, and the latency will be 10 - 20% higher. By default, the Go HTTP client will only keep two idle connections per each host.
 
 Keys which denote a duration are strings that can end with `s` or `m` to indicate seconds or minutes respectively. All of the other keys are integers. Supported keys are: