buffer: drop a reference to a non-existing function from the docs

This function only ever existed in the draft versions of the API.

libavutil/buffer.h

@@ -265,7 +265,6 @@ AVBufferPool *av_buffer_pool_init2(int size, void *opaque,
  * in use.
  *
  * @param pool pointer to the pool to be freed. It will be set to NULL.
- * @see av_buffer_pool_can_uninit()
  */
 void av_buffer_pool_uninit(AVBufferPool **pool);