chore(readme): correct heading level for fullScreenSwipeShadowEnabled (#2920)

- corrected heading level for `fullScreenSwipeShadowEnabled`

Author: donni106

native-stack/README.md

@@ -94,7 +94,7 @@ Possible options:
 
 Boolean indicating whether the swipe gesture should work on whole screen. Swiping with this option results in the same transition animation as `simple_push` by default. It can be changed to other custom animations with `customAnimationOnSwipe` prop, but default iOS swipe animation is not achievable due to usage of custom recognizer. Defaults to `false`.
 
-### `fullScreenSwipeShadowEnabled` (iOS only)
+#### `fullScreenSwipeShadowEnabled` (iOS only)
 
 Boolean indicating whether the full screen dismiss gesture has shadow under view during transition. The gesture uses custom transition and thus
 doesn't have a shadow by default. When enabled, a custom shadow view is added during the transition which tries to mimic the