Fix example file causing panic

[umbynos]

Having an example file in the library was causing the tool to panic, because it was being treated like a directory, but without checking if it was actually one.
The example file is just ignored now.
Close #751
The bug was discovered in arduino/library-registry#4836

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.05%. Comparing base (92436f3) to head (a4d7dd9).
Report is 17 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #752   +/-   ##
=======================================
  Coverage   90.05%   90.05%           
=======================================
  Files          44       44           
  Lines        6800     6800           
=======================================
  Hits         6124     6124           
  Misses        553      553           
  Partials      123      123           

Flag	Coverage Δ
unit	90.05% <100.00%> (ø)

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