Docs: typo in C++ serving example

[chenzhuofu]

The example command

./inference/spec_infer/spec_infer -ll:gpu 4 -ll:fsize 14000 -ll:zsize 30000 -llm-model meta-llama/Llama-2-7b-hf -ssm-model JackFram/llama-68m -prompt /path/to/prompt.json -tensor-parallelism-degree 4 --fusion

misses -ll:cpu 4 which will result in stuck before executing legion task background_serving_task.