resolve an issue in the developing-load-manager doc (apache#6978)

### Motivation

Resolves an issue in the load-manager docs

### Modifications

- added "broker" in the shell command for updating loadManagerClassName so that it works.

site2/website/versioned_docs/version-2.2.0/developing-load-manager.md

@@ -15,7 +15,7 @@ There are two ways that you can enable the modular load manager:
 2. Using the `pulsar-admin` tool. Here's an example:
 
    ```shell
-   $ pulsar-admin update-dynamic-config \
+   $ pulsar-admin brokers update-dynamic-config \
      --config loadManagerClassName \
      --value org.apache.pulsar.broker.loadbalance.impl.ModularLoadManagerImpl
    ```