Fix incorrect proxy_servers mention in the config reference docs

[ryanclark]

proxy_servers isn't a thing, it's proxy_server. This will fix the typo I made in #15761.

I've also reworded the line to be a bit clearer and true to the original it replaced. This makes the actual diff

     # Auth Server address and port to connect to. If you enable the Teleport
     # Auth Server to run in High Availability configuration, the address should
     # point to a Load Balancer.
-    # If adding a node located behind NAT, use the Proxy URL. e.g.
-    #  auth_servers:
-    #     - teleport-proxy.example.com:443
+    # If adding a node located behind NAT, use the Proxy URL (e.g. teleport-proxy.example.com:443)
+    # and set `proxy_server` instead.
+    auth_server: 10.1.0.5:3025

[ryanclark]

@ptgott this doesn't need backporting to v10 (as we're not backporting config v3 to v10, it's new from v11)