docs(modal): update fullscreen context to updated widthScale prop (#8313

)

**Related Issue:** #8250

## Summary
Updates the `modal`'s `fullscreen` prop context to the updated
`widthScale` prop (previously named `scale`).

packages/calcite-components/src/components/modal/modal.tsx

@@ -130,7 +130,7 @@ export class Modal
   /** Specifies the width of the component. */
   @Prop({ reflect: true }) widthScale: Scale = "m";
 
-  /** Sets the component to always be fullscreen (overrides `width` and `--calcite-modal-width` / `--calcite-modal-height`). */
+  /** Sets the component to always be fullscreen (overrides `widthScale` and `--calcite-modal-width` / `--calcite-modal-height`). */
   @Prop({ reflect: true }) fullscreen: boolean;
 
   /** Specifies the kind of the component (will apply to top border). */