chore: update image urls in readme

[darrenvechain]

Description

Update readme URLs: https://github.com/vechain/thor/tree/darren/chore/update-readme

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of 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
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• New and existing E2E tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I have not added any vulnerable dependencies to my code

[vanja-vechain]

LGTM!

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 61.73%. Comparing base (df93bff) to head (fefd49c).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #982      +/-   ##
==========================================
+ Coverage   61.71%   61.73%   +0.02%     
==========================================
  Files         218      218              
  Lines       22718    22718              
==========================================
+ Hits        14020    14025       +5     
+ Misses       7581     7578       -3     
+ Partials     1117     1115       -2     

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