Update hrmp commands zombienet (#2062)

The arguments need underscore in stead of capital letters (reference: https://substrate.stackexchange.com/questions/9364/debugging-zombienet-config-after-using-hrmp-channels)

content/md/en/docs/test/set-up-a-test-network.md

@@ -312,14 +312,14 @@ To set up communication between the parachains in the test network:
    [[hrmpChannels]]
    sender = 1000
    recipient = 1001
-   maxCapacity = 8
-   maxMessageSize = 8000
+   max_capacity = 8
+   max_message_size = 8000
 
    [[hrmpChannels]]
    sender = 1001
    recipient = 1000
-   maxCapacity = 8
-   maxMessageSize = 8000
+   max_capacity = 8
+   max_message_size = 8000
    ```
 
    Note that the values you set for **maxCapacity** and **maxMessageSize** shouldn't exceed the values defined for the `hrmpChannelMaxCapacity` and `hrmpChannelMaxMessageSize` parameters for the relay chain.