Add another example message to index DL3 files

[chaimain]

To help understand #1021

[codecov]

Codecov Report

Base: 71.64% // Head: 71.65% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (806e26f) compared to base (e5929be).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1034   +/-   ##
=======================================
  Coverage   71.64%   71.65%           
=======================================
  Files         120      120           
  Lines       11298    11301    +3     
=======================================
+ Hits         8095     8098    +3     
  Misses       3203     3203           

Impacted Files	Coverage Δ
lstchain/tools/lstchain_create_dl3_index_files.py	80.76% <ø> (ø)
lstchain/scripts/lstchain_dl1ab.py	82.35% <0.00%> (+0.31%)	⬆️

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