add missing .collapsible-list class

articles/components/date-time-picker/index.adoc

@@ -150,6 +150,7 @@ endif::[]
 --
 
 
+[.collapsible-list]
 == Validation
 
 Date Time Picker provides a validation mechanism based on constraints. Constraints allow you to define criteria that the date must meet to be considered valid. Validation typically occurs when the user initiates a date change, for example by selecting a date from the overlay or through text input followed by kbd:[Enter]. If the date is invalid, the field is visually highlighted in red, and an error message appears underneath the input.