fix(loadtest): correct the Docker build-context path in the README

The move to tools/ updated the Setup section but missed the Docker
section, which runs `cd loadtest && docker build ...`. From the repo root
that now fails.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: jmelahman

tools/loadtest/README.md

@@ -261,7 +261,7 @@ p95 / failure rate bend up, and which resource saturates first.
 ## Docker
 
 ```bash
-cd loadtest && docker build -f mock_llm/Dockerfile -t onyx-mock-llm .
+cd tools/loadtest && docker build -f mock_llm/Dockerfile -t onyx-mock-llm .
 docker run -p 8001:8000 onyx-mock-llm
 
 # Locust harness image (locustfile + scenarios baked in, for k8s/)