Update database.yml to include main: syntax

doc/install/installation.md

@@ -557,6 +557,7 @@ sudo -u git cp config/database.yml.postgresql config/database.yml
 # Once modified, the `production` settings will be as follows:
 #
 #   production:
+#     main:
 #       adapter: postgresql
 #       encoding: unicode
 #       database: gitlabhq_production