Fix spelling mistake and add section heaeder

[danielballan]

Follow-up PR. @mrakitin caught a typo. Also I think it might help to put a header on the paragraph about fidelity. I want to the "how-to" aspect to stand alone, looking short and easy. :-) Then the discussion can sit by itself.

In principal this cloud be moved to "Explanations" in the spirit of Diataxis, but I think it's pragmatic to keep it together in this case.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 67.93%. Comparing base (684e161) to head (aac5ccf).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #15   +/-   ##
=======================================
  Coverage   67.93%   67.93%           
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  Files          14       14           
  Lines        1809     1809           
=======================================
  Hits         1229     1229           
  Misses        580      580           

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[genematx]

I agree, mentioning this in Explanations would be very sensible, but I think it wouldn't hurt to have this in both places, actually.