Add port parameter to the sample config for psycopg2 args (#9911)

Adds the `port` option with the default value to the sample config file.

changelog.d/9911.doc

@@ -0,0 +1 @@
+Add `port` argument to the Postgres database sample config section.

docs/sample_config.yaml

@@ -810,6 +810,7 @@ caches:
 #    password: secretpassword
 #    database: synapse
 #    host: localhost
+#    port: 5432
 #    cp_min: 5
 #    cp_max: 10
 #

synapse/config/database.py

@@ -58,6 +58,7 @@ DEFAULT_CONFIG = """\
 #    password: secretpassword
 #    database: synapse
 #    host: localhost
+#    port: 5432
 #    cp_min: 5
 #    cp_max: 10
 #