Add AXIOM_URL to Syslog Proxy (#245)

Author: tothmano

send-data/syslog-proxy.mdx

@@ -89,6 +89,7 @@ Set the following environment variables to connect to Axiom:
 
 - `AXIOM_TOKEN` is the Axiom API token you have generated.
 - `AXIOM_DATASET` is the name of the Axiom dataset where you want to send data.
+- Optional: `AXIOM_URL` is the URL of the Axiom API. Set this environment variable if you want to use something other than the default URL `https://api.axiom.co/`.
 
 ## Run Axiom Syslog Proxy