Fix documentation of default EventIdSeparator

[304NotModified]

• Fix documentation or default
• move init to properties (so it's better visible)

[codecov-commenter]

Codecov Report

Merging #429 into master will decrease coverage by 0.08%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #429      +/-   ##
==========================================
- Coverage   82.30%   82.22%   -0.09%     
==========================================
  Files          14       14              
  Lines        1204     1198       -6     
  Branches      200      200              
==========================================
- Hits          991      985       -6     
  Misses        141      141              
  Partials       72       72              

Impacted Files	Coverage Δ
....Extensions.Logging/Logging/NLogProviderOptions.cs	100.00% <100.00%> (ø)

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[snakefoot]

Looks great