update ports since PR mimblewimble#2405. We now allow both floo and mainnet owner_apis to run in parallell on a single server, with ports 13420 or 3420

Author: sesam

Diff for: doc/api/api.md

@@ -17,6 +17,9 @@ To learn about what specific calls can be made read the [wallet foreign API doc]
 
 ### Wallet Owner API
 
-The wallet owner API is an endpoint to manage the user wallet: broadcast transaction, sign transaction, see the current balance... This REST API can be started with the `grin wallet owner_api` command and will listen on `localhost:13420`. This endpoint must **never** be exposed to the outside world.
+The wallet owner API is an endpoint to manage the user wallet: broadcast transaction, sign transaction, see the current balance... This REST API can be started with the `grin wallet owner_api` command and will listen on `localhost:13420` for --floonet or `localhost:3420` for --mainnet.
+
+__This endpoint must **never** be exposed to the outside world.__
+
 This endpoint requires, by default, Basic Authentication. The username is `grin` and the password can be found in the `.api_secret` file.
 To learn about what specific calls can be made read the [wallet owner API doc](wallet_owner_api.md).