Correct syntax for setting skin.

[pkreissl]

Shinx documentation:
Fixes the syntax in sample for setting skin in the introduction.
This line lead to some confusion at this year's SFB summer school.

[codecov]

Codecov Report

Merging #2340 into python will increase coverage by <1%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           python   #2340    +/-   ##
=======================================
+ Coverage      71%     71%   +<1%     
=======================================
  Files         381     381            
  Lines       18915   18915            
=======================================
+ Hits        13566   13587    +21     
+ Misses       5349    5328    -21

Impacted Files	Coverage Δ
src/core/layered.cpp	79% <0%> (ø)	⬆️
src/core/tuning.cpp	36% <0%> (+2%)	⬆️
src/core/domain_decomposition.cpp	91% <0%> (+3%)	⬆️
src/core/lattice.cpp	89% <0%> (+6%)	⬆️

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