Add directory picker example to file_dialog.py

[GenevieveBuckley]

I thought it would be helpful to add a directory picker example to examples/file_dialog.py
People might be unaware that's a thing you can do, so it would be good to highlight it as well.

[codecov]

Codecov Report

Base: 89.88% // Head: 89.90% // Increases project coverage by +0.02% 🎉

Coverage data is based on head (1946a7f) compared to base (a6b9b70).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #524      +/-   ##
==========================================
+ Coverage   89.88%   89.90%   +0.02%     
==========================================
  Files          38       38              
  Lines        4437     4437              
==========================================
+ Hits         3988     3989       +1     
+ Misses        449      448       -1     

Impacted Files	Coverage Δ
src/magicgui/widgets/_concrete.py	89.05% <0.00%> (+0.21%)	⬆️

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[tlambert03]

Thanks!