erp5_short_message: update description, this business template is no longer experimental

bt5/erp5_short_message/bt/description

-Experimental BT to provide Short Message Service in ERP5
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+Provides Short Message Service in ERP5
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[Jérome Perrin]

We can remove the "Experimental" statement, but unless I am mistaken, we still do not have tests covering portal_sms