docs: Fix training script path in README_TRAIN.md

[harikapadia999]

Description

Fixes inconsistent training script path reference in README_TRAIN.md

Problem

The first paragraph of README_TRAIN.md incorrectly states the training script is located at sam3/train.py, but the actual location is sam3/train/train.py.

Changes

• Updated line 3: Changed sam3/train.py → sam3/train/train.py
• This makes the documentation consistent with:
  • The actual file location in the repository
  • All usage examples throughout the document which correctly use sam3/train/train.py

Impact

• Improves documentation accuracy
• Prevents confusion for new users trying to locate the training script
• Maintains consistency throughout the training documentation

Checklist

• [x] Documentation-only change
• [x] No code changes
• [x] Improves accuracy and consistency