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

Documentation/bpf/instruction-set.rst

@@ -122,11 +122,11 @@ BPF_END   0xd0   byte swap operations (see `Byte swap instructions`_ below)
 
 ``BPF_XOR | BPF_K | BPF_ALU`` means::
 
-  src_reg = (u32) src_reg ^ (u32) imm32
+  dst_reg = (u32) dst_reg ^ (u32) imm32
 
 ``BPF_XOR | BPF_K | BPF_ALU64`` means::
 
-  src_reg = src_reg ^ imm32
+  dst_reg = dst_reg ^ imm32
 
 
 Byte swap instructions