Fix image links in Blender procedural generation tutorial

[mabelzhang]

🦟 Bug fix

Summary

Images in the Blender procedural generation tutorial are not showing up on the rendered page (they only show up in GitHub)
https://gazebosim.org/api/sim/7/blender_procedural_datasets.html

Need to use raw image links.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[codecov]

Codecov Report

Merging #1996 (fcc6be4) into gz-sim7 (42c285e) will not change coverage.
The diff coverage is n/a.

❗ Current head fcc6be4 differs from pull request most recent head 0e90384. Consider uploading reports for the commit 0e90384 to get more accurate results

@@           Coverage Diff            @@
##           gz-sim7    #1996   +/-   ##
========================================
  Coverage    64.97%   64.97%           
========================================
  Files          353      353           
  Lines        28600    28600           
========================================
  Hits         18584    18584           
  Misses       10016    10016           

see 2 files with indirect coverage changes