qemu-img: fix rebase src_cache option documentation

qemu-img: fix rebase src_cache option documentation

The src_cache option (-T) specifies the cache mode for backing files.
It applies both the image's old backing file as well as the new backing
file:

  ret = bdrv_open(&bs_old_backing, backing_name, NULL, NULL, src_flags,
                  old_backing_drv, &local_err);
  if (ret) {
      ...
  }
  if (out_baseimg[0]) {
      bs_new_backing = bdrv_new("new_backing", &error_abort);
      ret = bdrv_open(&bs_new_backing, out_baseimg, NULL, NULL, src_flags,
                      new_backing_drv, &local_err);
      if (ret) {
          ...
      }
  }

The documentation only mentions the new backing file but it really
applies to both.

Suggested-by: Jeff Nelson <jenelson@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>