SDL_BlitSurface() comment: only dstrect is modified in function

(cherry picked from commit 035518d06502e144e3e52208adf8d0fb036194d8)

include/SDL_surface.h

@@ -808,8 +808,8 @@ extern DECLSPEC int SDLCALL SDL_FillRects
  *
  * This assumes that the source and destination rectangles are the same size.
  * If either `srcrect` or `dstrect` are NULL, the entire surface (`src` or
- * `dst`) is copied. The final blit rectangles are saved in `srcrect` and
- * `dstrect` after all clipping is performed.
+ * `dst`) is copied. The final blit rectangle is saved in `dstrect` after
+ * all clipping is performed.
  *
  * The blit function should not be called on a locked surface.
  *