libbpf: Fix license in README.rst

The whole libbpf is licensed as (LGPL-2.1 OR BSD-2-Clause). I missed it
while adding README.rst. Fix it and use same license as all other files
in libbpf do. Since I'm the only author of README.rst so far, no others'
permissions should be needed.

Fixes: 76d1b894 ("libbpf: Document API and ABI conventions")
Signed-off-by: NAndrey Ignatov <rdna@fb.com>
Acked-by: NSong Liu <songliubraving@fb.com>
Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net>

tools/lib/bpf/README.rst

-.. SPDX-License-Identifier: GPL-2.0
+.. SPDX-License-Identifier: (LGPL-2.1 OR BSD-2-Clause)
 
 libbpf API naming convention
 ============================