Exercise 4 and 5 notebooks have 'copy' in filenames

[schwentker]

Two exercise notebooks have "copy" appended to their filenames, which appears to be unintentional:

Current filenames:

• 4-Flexible_extraction_with_extract copy.ipynb
• 5-Structured_extraction_with_extract_structured copy.ipynb

Should be:

• 4-Flexible_extraction_with_extract.ipynb
• 5-Structured_extraction_with_extract_structured.ipynb

This creates inconsistency with the other exercise files and may confuse users following the workshop sequence.