Fix: agent container migration path (#940)

Author: john-david3

content/includes/agent/installation/update-container.md

@@ -10,7 +10,7 @@ To migrate NGINX Agent containers, we provide a script to convert NGINX Agent v2
 To upgrade the configuration, you can follow this example:
 
 ```shell
-wget https://raw.githubusercontent.com/nginx/agent/refs/heads/v3/scripts/packages/upgrade-agent-config.sh
+wget https://raw.githubusercontent.com/nginx/agent/refs/heads/main/scripts/packages/upgrade-agent-config.sh
 ./upgrade-agent-config.sh --v2-config-file=./nginx-agent-v2.conf --v3-config-file=nginx-agent-v3.conf
 ```