Added POSTGRES_PASSWORD to fix example code

The existing example code for the postgres container implementation of Kong is broken. 

Figured out that adding environmental variables POSTGRES_PASSWORD and KONG_PG_PASSWORD to the command arguments fixes the issue.

kong/content.md

@@ -33,6 +33,7 @@ $ docker run -d --name kong-database \
                 -p 5432:5432 \
                 -e "POSTGRES_USER=kong" \
                 -e "POSTGRES_DB=kong" \
+                -e "POSTGRES_PASSWORD=kong" \
                 postgres:9.6
 ```
 
@@ -45,6 +46,8 @@ $ docker run --rm \
     --link kong-database:kong-database \
     -e "KONG_DATABASE=postgres" \
     -e "KONG_PG_HOST=kong-database" \
+    -e "KONG_PG_USER=kong" \
+    -e "KONG_PG_PASSWORD=kong" \
     -e "KONG_CASSANDRA_CONTACT_POINTS=kong-database" \
     %%IMAGE%% kong migrations bootstrap
 ```
@@ -62,6 +65,7 @@ $ docker run -d --name kong \
     --link kong-database:kong-database \
     -e "KONG_DATABASE=postgres" \
     -e "KONG_PG_HOST=kong-database" \
+    -e "KONG_PG_PASSWORD=kong" \
     -e "KONG_CASSANDRA_CONTACT_POINTS=kong-database" \
     -e "KONG_PROXY_ACCESS_LOG=/dev/stdout" \
     -e "KONG_ADMIN_ACCESS_LOG=/dev/stdout" \