Fix potential 502 errors when using Gitlab Agent helm chart

In Zendesk ticket 310232, customer reports the following errors from their Kubernetes Agents:

{"level":"error","time":"2022-07-02T07:29:14.944Z","msg":"Error handling a connection","mod_name":"reverse_tunnel","error":"Connect(): rpc error: code = Unavailable desc = connection error: desc = \"transport: Error while dialing failed to WebSocket dial: expected handshake response status code 101 but got 502\""}

Our Troubleshooting Documentation mentions:

This error occurs when the kas-address doesn’t include a trailing slash. To fix this issue, make sure that the wss or ws URL ends with a trailing slash, like wss://GitLab.host.tld:443/-/kubernetes-agent/ or ws://GitLab.host.tld:80/-/kubernetes-agent/.

The customer tested this and confirmed it seemed to resolve the errors for them. So, this MR adds the trailing slash to the helm chart to avoid this error in the future.