Fix wrong info regarding custom left buttons in documentation

website/docs/docs-stack.mdx

 
 ### Using a react component in a button
 :::caution
-At the moment, custom buttons in `rightButtons` are supported only on iOS.
+At the moment, custom buttons in `leftButtons` are supported only on iOS.
 :::
 
 Sometimes we require more from our buttons. In order to support every product need React Components can be used as custom views of buttons.