normalize index column name in subarray table

[kosack]

Just a simple change suggested in #1059 to change the column 'id' in the output of SubarrayDescription.to_table() to be tel_id

[codecov]

Codecov Report

Merging #1065 into master will increase coverage by 0.14%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1065      +/-   ##
==========================================
+ Coverage   83.27%   83.41%   +0.14%     
==========================================
  Files         181      181              
  Lines       10415    10416       +1     
==========================================
+ Hits         8673     8689      +16     
+ Misses       1742     1727      -15

Impacted Files	Coverage Δ
ctapipe/instrument/tests/test_subarray.py	97.22% <100%> (+0.07%)	⬆️
ctapipe/instrument/subarray.py	94.94% <100%> (+15.15%)	⬆️
ctapipe/tools/display_summed_images.py	91.07% <100%> (ø)	⬆️

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[thomasgas]

If it doesn't have to be changed anywhere else, I'm fine.