docs: replace dead Adobe TIFF spec links with libtiff mirrors

[FirmaSpring]

Problem

Three reference links in src/ImageSharp/Formats/Tiff/README.md point at retired partners.adobe.com URLs. Each returns GET 404:

curl -sL -o /dev/null -w "%{http_code}\n" http://partners.adobe.com/public/developer/en/tiff/TIFF6.pdf
# 404
curl -sL -o /dev/null -w "%{http_code}\n" http://partners.adobe.com/public/developer/en/tiff/TIFFPM6.pdf
# 404
curl -sL -o /dev/null -w "%{http_code}\n" http://partners.adobe.com/public/developer/en/tiff/TIFFphotoshop.pdf
# 404

The line for TIFF 6.0 also had a second bare URL appended as (http://www.npes.org/pdf/TIFF-v6.pdf) which was not a real markdown link.

Solution

Point all three at the libtiff project mirrors on download.osgeo.org, which host the same specification PDFs:

• https://download.osgeo.org/libtiff/doc/TIFF6.pdf
• https://download.osgeo.org/libtiff/doc/TIFFPM6.pdf
• https://download.osgeo.org/libtiff/doc/TIFFphotoshop.pdf

All three verified 200 with curl -L GET.

Verification

Markdown-only change. Searched open and closed PRs for prior TIFF specification link fixes and found none.