docs: Update mqtt-namespace.md (#5081)

Matching [code](https://github.com/eclipse/kura/blob/a73c7af6c1d08872c7b5b1aeba544eca0ea09601/kura/org.eclipse.kura.core.inventory/src/main/java/org/eclipse/kura/core/inventory/InventoryHandlerV1.java#L74) and respective documentation for REST APIs [here](https://eclipse.github.io/kura/docs-release-5.4/references/rest-apis/rest-inventory-api/#get-system-packages)

(cherry picked from commit 2d6fd4e)

docs/references/mqtt-namespace.md

@@ -936,7 +936,7 @@ The deployment package JSON message is comprised of the following package elemen
 This operation provides the Linux packages installed in OS.
 
 * Request Topic:
-    * **$EDC/account_name/client_id/INVENTORY-V1/GET/system.packages**
+    * **$EDC/account_name/client_id/INVENTORY-V1/GET/systemPackages**
 
 * Request Payload:
     * Nothing application-specific beyond the request ID and requester client ID