[IMP] rdtraining: update float utils information

The warning about float utils does not state where to find them, and
they are preceded by an example in the code that infringes the warning's
recommendation.

This commit indicates where to find the float utils and replaces the
example by one that uses them.

task-2925402

X-original-commit: b8ee134a28

content/developer/howtos/rdtraining/11_constraints.rst

@@ -101,7 +101,7 @@ raise an exception if its invariant is not satisfied::
         # all records passed the test, don't return anything
 
 A simple example can be found
-`here <https://github.com/odoo/odoo/blob/3783654b87851bdeb11e32da78bb5b62865b869a/addons/account/models/account_payment_term.py#L104-L108>`__.
+`here <https://github.com/odoo/odoo/blob/274dd3bf503e1b612179db92e410b336bfaecfb4/addons/stock/models/stock_quant.py#L239-L244>`__.
 
 .. exercise:: Add Python constraints.
 
@@ -113,7 +113,8 @@ A simple example can be found
     .. warning::
 
         Always use the :meth:`~odoo.tools.float_utils.float_compare` and
-        :meth:`~odoo.tools.float_utils.float_is_zero` methods when working with floats!
+        :meth:`~odoo.tools.float_utils.float_is_zero` methods from `odoo.tools.float_utils` when
+        working with floats!
 
     Ensure the constraint is triggered every time the selling price or the expected price is changed!