Update POSTGRESQL.md

Step 3 was using the deprecated dius/pact_broker image (with underscore instead of dash).
Updated to use dius/pact-broker instead.

POSTGRESQL.md

@@ -27,7 +27,7 @@ The best way is to run postgresql via docker as well.
 3. Start the PactBroker container via:
 
     ```console
-    $ docker run --name pactbroker --link pactbroker-db:postgres -e PACT_BROKER_DATABASE_USERNAME=pactbrokeruser -e PACT_BROKER_DATABASE_PASSWORD=TheUserPassword -e PACT_BROKER_DATABASE_HOST=postgres -e PACT_BROKER_DATABASE_NAME=pactbroker -d -p 80:80 dius/pact_broker
+    $ docker run --name pactbroker --link pactbroker-db:postgres -e PACT_BROKER_DATABASE_USERNAME=pactbrokeruser -e PACT_BROKER_DATABASE_PASSWORD=TheUserPassword -e PACT_BROKER_DATABASE_HOST=postgres -e PACT_BROKER_DATABASE_NAME=pactbroker -d -p 80:80 dius/pact-broker
     ```
 
 4. (Don't need to run this) Finally if you want to reconfigure/remove the container you will need to use