[FIX] orm: string pattern for Reference wasn't correct

In reality the separator is a comma instead of a dot as can be seen
in the code a bit further below.

closes odoo/odoo#85396

X-original-commit: 2a6e9f166d493aaefc006e949ab45e42311ac886
Signed-off-by: Xavier Morel (xmo) <xmo@odoo.com>

odoo/fields.py

@@ -2623,7 +2623,7 @@ class Reference(Selection):
     """ Pseudo-relational field (no FK in database).
 
     The field value is stored as a :class:`string <str>` following the pattern
-    ``"res_model.res_id"`` in database.
+    ``"res_model,res_id"`` in database.
     """
     type = 'reference'