docs, bpf: fix code-block for load XDP BPF example
docs, bpf: fix code-block for load XDP BPF example

On load XDP BPF example section at "BPF and XDP Reference Guide"
there are minor formatting issues related to code-block.
Thus, this commit fixes these minor formatting issues.

Signed-off-by: Claudia J. Kang <0c52aea8aae24dfb42bbd953e4640dc55d314128@gmail.com>

tklauser/cilium,cilium/cilium,cilium/cilium,michi-covalent/cilium,michi-covalent/cilium,michi-covalent/cilium,cilium/cilium,tklauser/cilium,michi-covalent/cilium,michi-covalent/cilium,tklauser/cilium,cilium/cilium,tklauser/cilium,cilium/cilium,tklauser/cilium