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 :white_check_mark:

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

Additional details and impacted files

```diff @@ 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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