bpf, docs: Correct the example of BPF_XOR

Refer to description of BPF_XOR, dst_reg should be used but not src_reg
in the examples.

Fixes: be3193cd ("bpf, docs: Add subsections for ALU and JMP instructions")
Signed-off-by: Zheng Yejian <zhengyejian1@huawei.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20221129134558.2757043-1-zhengyejian1@huawei.com