message: Update extension use in README
diff: @@ -36,7 +36,7 @@ let instance = entry.create_instance(&create_info, None)
  ### `Vec<T>` instead of mutable slices
  ```Rust
  -pub fn get_swapchain_images_khr(&self,
  +pub fn get_swapchain_images(&self,
  swapchain: vk::SwapchainKHR)
  -> VkResult<Vec<vk::Image>>;
  let present_images = swapchain_loader.get_swapchain_images_khr(swapchain).unwrap();
  @@ -123,9 +123,9 @@ Custom loaders can be implemented.
  ### Extension loading
  Additionally, every Vulkan extension has to be loaded explicitly. You can find all extensions under [ash::extensions](https://github.com/MaikKlein/ash/tree/master/ash/src/extensions).
  ```Rust
  -use ash::extensions::Swapchain;
  +use ash::extensions::khr::Swapchain;
  let swapchain_loader = Swapchain::new(&instance, &device);
  -let swapchain = swapchain_loader.create_swapchain_khr(&swapchain_create_info).unwrap();
  +let swapchain = swapchain_loader.create_swapchain(&swapchain_create_info).unwrap();
  ```
  ### Raw function pointers
  