Fix documentation for running stand alone network (#2502)

The documentation stated that the root account seed was configured with the `NODE_SEED` variable in the stellar core config. That is incorrect. The `NODE_SEED` variable configures the seed for the validator account. The root account which holds the lumen supply is derived from the network passphrase and is printed in the stellar core logs.

services/horizon/docker/README.md

@@ -82,11 +82,14 @@ Stellar public network, run `docker-compose -f docker-compose.yml -f docker-comp
 
 To run the containers on a private stand alone network, run `docker-compose -f docker-compose.yml -f docker-compose.standalone.yml up -d --build`.
 When you run Stellar Core on a private stand alone network, an account will be created which will hold 100 billion Lumens.
-The seed for the account can be found in [stellar-core-standalone.cfg](./stellar-core-standalone.cfg) (see `NODE_SEED` parameter). 
-The seed is also emitted in the Stellar Core logs.
+The seed for the account will be emitted in the Stellar Core logs:
+
+```
+2020-04-22T18:39:19.248 GD5KD [Ledger INFO] Root account seed: SC5O7VZUXDJ6JBDSZ74DSERXL7W3Y5LTOAMRF7RQRL3TAGAPS7LUVG3L
+```
 
 When you switch between different networks you will need to clear the Stellar Core and Stellar Horizon databases. You can wipe out the databases by running `docker-compose down -v`.
 
 ## Using a specific version of Stellar Core
 
-By default the Docker Compose file is configured to use the latest version of Stellar Core. To use a specific version, you can edit [docker-compose.yml](./docker-compose.yml) and set the appropriate [tag](https://hub.docker.com/r/stellar/stellar-core/tags) on the Stellar Core docker image
+By default the Docker Compose file is configured to use the latest version of Stellar Core. To use a specific version, you can edit [docker-compose.yml](./docker-compose.yml) and set the appropriate [tag](https://hub.docker.com/r/stellar/stellar-core/tags) on the Stellar Core docker image