[FIX] web_editor: spacing when changing format in checklists

Before this commit:

On changing style of a list item to normal it wrapped the content in a <p> which
results in extra spacing to next list item.

After this commit:

Now the content is not wrapped in a <p> which removes the extra spacing as well.

Task-2990517

closes odoo/odoo#102537

Signed-off-by: David Monjoie (dmo) <dmo@odoo.com>