[Docs] Fix ghost --kv-cache-memory flag in optimization guide

[imitater-dou]

Summary

• docs/configuration/optimization.md told users to pass --kv-cache-memory to skip memory profiling.
• The live vllm serve CLI only registers --kv-cache-memory-bytes (vllm/engine/arg_utils.py); there is no --kv-cache-memory alias, so the documented flag is a ghost.
• Update both references in that bullet to --kv-cache-memory-bytes.

Repro

1. Docs (HEAD 782f36cd0c7951cc1fa3c4fb3eeb916cc31780ed): docs/configuration/optimization.md line ~24 uses `--kv-cache-memory` twice.
2. Code: arg_utils.py registers only --kv-cache-memory-bytes for CacheConfig.kv_cache_memory_bytes.
3. Passing --kv-cache-memory=... as written in the docs is not a valid CLI option.

Signed-off-by: ADou ikun3.1415927@gmail.com

[mergify]

Documentation preview: https://vllm--55834.org.readthedocs.build/en/55834/

[github-actions]

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