include: fixed a typo in SDL_RenderGetMetalCommandEncoder docs.

(cherry picked from commit 8087c0a5d2e2e47b7841cdd9d3fdff8cfb23e241)

include/SDL_render.h

@@ -1890,7 +1890,7 @@ extern DECLSPEC void *SDLCALL SDL_RenderGetMetalLayer(SDL_Renderer * renderer);
  * Note that as of SDL 2.0.18, this will return NULL if Metal refuses to give
  * SDL a drawable to render to, which might happen if the window is
  * hidden/minimized/offscreen. This doesn't apply to command encoders for
- * render targets, just the window's backbacker. Check your return values!
+ * render targets, just the window's backbuffer. Check your return values!
  *
  * \param renderer The renderer to query
  * \returns an `id<MTLRenderCommandEncoder>` on success, or NULL if the