docs: fix stale --tp flag in bench SGLang examples

[imitater-dou]

Summary

Replace invalid --tp with --tp-size in the SGLang launch_server examples under benchmarks/README.md and the benchmarks/bench_eagle3.py usage docstring.

SGLang 0.5.18 ServerArgs exposes --tp-size (alias --tensor-parallel-size), not --tp. The corrected examples match docs/benchmarks/benchmark.md and the flags bench_eagle3.py already forwards when launching a server.

How verified

Against upstream sgl-project/SpecForge main SHA 2fc993077c3c53df14ebdd5d414c854864476af8 and SGLang v0.5.18 ServerArgs:

• tp_size CLI flag: --tp-size with alias --tensor-parallel-size only (no --tp)
• SpecForge pin: sglang==0.5.18 in pyproject.toml
• docs/benchmarks/benchmark.md already uses --tp-size 1
• bench_eagle3.py launch path already passes "--tp-size" from server_args.tp_size

Before

python3 -m sglang.launch_server \
  ...
  --tp 1 \
  ...

After

python3 -m sglang.launch_server \
  ...
  --tp-size 1 \
  ...