[LLM:Doc] Fix Web LLM demo source path

[eyesofish]

Summary

• update the Web LLM demo compile command to reference llm_demo.cpp in its current engine/demo directory
• prevent the documented Emscripten command from failing because the old source path no longer exists

Verification

• confirmed transformers/llm/engine/demo/llm_demo.cpp exists
• confirmed the old transformers/llm/engine/llm_demo.cpp path does not exist
• ran git diff --check

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