Update diagnostics-container.md

I changed the sample code to have a lowercase "eula" instead of "Eula" since otherwise the command fails stating that the eula was not accepted. 
I tested on Windows Server 2019 with WSL running this docker command from PowerShell.

Author: PaulHCode

articles/cognitive-services/containers/includes/diagnostics-container.md

@@ -21,9 +21,9 @@ Then run the container. Replace `{ENDPOINT_URI}` with your endpoint, and replace
 
 ```bash
 docker run --rm mcr.microsoft.com/azure-cognitive-services/diagnostic \
-Eula=accept \
+eula=accept \
 Billing={ENDPOINT_URI} \
 ApiKey={API_KEY}
 ```
 
-The container will test for network connectivity to the billing endpoint.
+The container will test for network connectivity to the billing endpoint.