docs(zip): fix slash names in CleanName comment

[ueli-werner]

I certify that I own, and have sufficient rights to contribute, all source code and related material intended to be compiled or integrated with the source code for the SharpZipLib open source product (the "Contribution"). My Contribution is licensed under the MIT License.

[piksel]

Simple typo fix. Not much to argue with. Thanks!

[codecov]

Codecov Report

Merging #668 (2ceb79c) into master (c086bc2) will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #668      +/-   ##
==========================================
+ Coverage   73.31%   73.32%   +0.01%     
==========================================
  Files          68       68              
  Lines        8305     8305              
==========================================
+ Hits         6089     6090       +1     
+ Misses       2216     2215       -1     

Impacted Files	Coverage Δ
src/ICSharpCode.SharpZipLib/Zip/ZipEntry.cs	90.90% <ø> (ø)
...Code.SharpZipLib/Zip/Compression/DeflaterEngine.cs	86.03% <0.00%> (+0.37%)	⬆️

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