ScanPDF: Modification of regex throwing warning

[phutelmyer]

Describe the change
Update phone_number regular expression in ScanPDF to fix warning observed during tests:

=============================== warnings summary ===============================
scanners/scan_pdf.py:14
  /strelka/strelka/scanners/scan_pdf.py:14: DeprecationWarning: invalid escape sequence '\+'
    "\+?(?:\d{1,2})?\s?\(?\d{3}\)?[\s.-]\d{3}[\s.-]\d{2,4}?\-?\d{2,4}?",

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
======================== 31 passed, 1 warning in 5.27s =========================

The updated regular expression will match phone numbers that are formatted in a variety of ways, including:

+1 (123) 456-7890
(123) 456-7890
123-456-7890
123 456 7890
123.456.7890

Describe testing procedures
Submitted several PDFs with phone numbers to Strelka.

Sample output
N/A

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of and tested my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings