Fixing caution

reference/en/transactions.txt

@@ -92,7 +92,7 @@ because the nested transaction blocks are in a third-party API you're out of luc
 All that is guaruanteed to the inner transaction is that it still happens atomically, all or nothing, the transaction just gets a wider scope
 and the control is handed to the outer scope.
 
-> *CAUTION*
+> **CAUTION**
 > The transaction nesting described here is a debated feature that has it's critics. Form your own opinion.
 > We recommend avoiding nesting transaction blocks when possible, and most of the time, it is possible.
 > Transaction control should mostly be left to a service layer and not be handled in data access objects or similar.