Include metricity db creation for local setup guide.

This is later mentioned in the connection string, but in the local setup, the
SQL statement to create the database was missing.

Author: jchristgit

pydis_site/apps/content/resources/guides/pydis-guides/contributing/site.md

@@ -62,6 +62,7 @@ Run the following queries to create the user and database:
 ```sql
 CREATE USER pysite WITH SUPERUSER PASSWORD 'pysite';
 CREATE DATABASE pysite WITH OWNER pysite;
+CREATE DATABASE metricity WITH OWNER pysite;
 ```
 
 Finally, enter `/q` to exit psql.