Added correct material.png figure in tool-instructions

[francescalb]

Description:

Correct Figure of the material branch from emmo-1.0.0beta4.
Also added a format requirement since it did not take the .png extension automatically from the name.
This should be fixed in a dfferent PR in respons to issue #497

Type of change:

• Bug fix.
• New feature.
• Documentation update.

Checklist:

This checklist can be used as a help for the reviewer.

• Is the code easy to read and understand?
• Are comments for humans to read, not computers to disregard?
• Does a new feature has an accompanying new test (in the CI or unit testing schemes)?
• Has the documentation been updated as necessary?
• Does this close the issue?
• Is the change limited to the issue?
• Are errors handled for all outcomes?
• Does the new feature provide new restrictions on dependencies, and if so is this documented?

Comments:

[codecov-commenter]

Codecov Report

Merging #498 (4d3fc7a) into master (64782c8) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #498   +/-   ##
=======================================
  Coverage   63.15%   63.15%           
=======================================
  Files          16       16           
  Lines        3078     3078           
=======================================
  Hits         1944     1944           
  Misses       1134     1134           

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