Clarified format of postgres_dsn (#7139)

* Clarified format of postgres_dsn

* Update source/deploy/postgres-migration.rst

Co-authored-by: Ibrahim Serdar Acikgoz <[email protected]>

* Update source/deploy/postgres-migration.rst

Co-authored-by: Ibrahim Serdar Acikgoz <[email protected]>

* Update source/deploy/postgres-migration.rst

Co-authored-by: Ibrahim Serdar Acikgoz <[email protected]>

---------

Co-authored-by: Ibrahim Serdar Acikgoz <[email protected]>

Author: cwarnermm

source/deploy/postgres-migration.rst

@@ -275,9 +275,13 @@ For our case, we can simply run the following command:
 
 .. code:: sh
 
-   dbcmp --source "${MYSQL_DSN}" --target "${POSTGRES_DSN}" --exclude="db_migrations,ir_,focalboard,systems"
+   dbcmp --source "${MYSQL_DSN}" --target "${POSTGRES_DSN} " --exclude="db_migrations,ir_,focalboard,systems"
 
-Note that this migration guide only covers the tables for Mattermost products.
+An example command would look like: ``dbcmp --source "user:password@tcp(address:3306)/db_name --target "postgres://user:password@address:5432/db_name``
+
+.. note::
+   
+   ``POSTGRES_DSN`` should start with a ``postgres://`` prefix. This way ``dbcmp`` decides which driver to use while connecting to a database.
 
 Another exclusion we are making is in the ``db_migrations`` table which has a small difference (a typo in a single migration name) and creates a diff. Since we created the PostgreSQL schema with morph, and the official ``mattermost`` source, we can skip it safely without concerns. On the other hand, ``systems`` table may contain additional diffs if there were extra keys added during some of the migrations. Consider excluding the ``systems`` table if you run into issues, and perform a manual comparison as the data in the ``systems`` table is relatively smaller in size.