Fix docs for collapsible preselected section

[MacGyer]

Proposed changes

The current docs state:

If you want a collapsible with a preopened section just add the active class to the collapsible-header.

It should say, that the active class has to be added to the <li> instead the collapsible-header.

Types of changes

• Documentation fix

Checklist:

• I have read the CONTRIBUTING document.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.