Fix hard-coded language in nllb_translation example.

[fumoboy007]

Proposed changes

The target language was accidentally hard-coded to yor_Latn (Yoruba) instead of being customizable by the --target_language command-line option.

Types of changes

• New Model Architecture
• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update (if none of the other choices apply)