Tweak material results example heading

[greschd]

As suggested in internal testing: the previous heading did not make it clear whether the results themselves are materials, or the results are selected per material.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.50%. Comparing base (a4185ca) to head (617b887).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #527   +/-   ##
=======================================
  Coverage   91.50%   91.50%           
=======================================
  Files          50       50           
  Lines        3356     3356           
=======================================
  Hits         3071     3071           
  Misses        285      285           

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