Correct init command for Postgres Database (#1243)

It seems that the MySQL syntax command was mistakenly copied to the
instructions for Postgres, which uses a different syntax - ref
[here](https://www.postgresql.org/docs/current/sql-createdatabase.html)
for creation syntax, and
[here](https://www.postgresql.org/docs/current/multibyte.html) for
encoding information. I did read through
[here](https://www.postgresql.org/docs/current/collation.html) looking
for an appropriate collation, but they appear to be
installation-specific rather than having a standard set defined in
documentation - I omitted this value (thus setting to the default), and
haven't noted any immediate problems.

Co-authored-by: Joey Orlando <joey.orlando@grafana.com>

helm/oncall/README.md

@@ -205,7 +205,7 @@ database:
   type: postgresql
 
 # Make sure to create the database with the following parameters:
-# CREATE DATABASE oncall CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci;
+# CREATE DATABASE oncall WITH ENCODING UTF8;
 externalPostgresql:
   host:
   port: