Incorrect environment variable name in RQ configuration docs

[Ulli2060]

Problem

The RQ engine configuration documentation contains a duplicate entry for DOCLING_SERVE_ENG_RQ_RESULTS_PREFIX where the second occurrence should be DOCLING_SERVE_ENG_RQ_SUB_CHANNEL.

Location

• File: docs/configuration.md
• Line: 80

Current (incorrect)

| `DOCLING_SERVE_ENG_RQ_RESULTS_PREFIX` | `docling:updates` | The channel key name... |

Fixed (correct)

| `DOCLING_SERVE_ENG_RQ_SUB_CHANNEL` | `docling:updates` | The channel key name... |

Verification

Checked against docling_serve/settings.py lines 71-73 which defines:

• eng_rq_redis_url
• eng_rq_results_prefix
• eng_rq_sub_channel

This fix ensures the documentation matches the actual code implementation.