Update property definition for backdrop in offcanvas component

Problem/Motivation

The README.mdx file for the offcanvas component lists the backdrop property as a boolean, but if it is used this way it will not work correctly as the logic for setting a default value depends on a string.

If false is passed to this component instead of 'false' the resulting value from the following code will be 'true'.
{% set backdrop = backdrop|default('true') %}

The comments in offcanvas.twig accurately define this:
* - backdrop: (optional) Backdrop: 'true' | 'false' | 'static'.

The definition in README.mdx shows:
- `backdrop` (bool) (default: true): Backdrop.

Proposed resolution

Update the README, defining this as a string instead of a bool.