Remove slash from PackagePath

[Ivanidzo4ka]

fixes #2255

[eerhardt]

Do these need to be fixed as well?

machinelearning/pkg/Microsoft.ML.TensorFlow.Redist/Microsoft.ML.TensorFlow.Redist.nupkgproj

Line 22 in cabf55b

<Content Include="$(PackageAssetsPath)$(PackageIdFolderName)\LICENSE.txt" Pack="true" PackagePath=".\" />

machinelearning/pkg/Microsoft.ML.Recommender/Microsoft.ML.Recommender.nupkgproj

Line 11 in da973ba

<Content Include="$(SourceDir)Native\MatrixFactorizationNative\libmf\COPYRIGHT" Pack="true" PackagePath=".\" />

machinelearning/pkg/Microsoft.ML.Mkl.Redist/Microsoft.ML.Mkl.Redist.nupkgproj

Line 17 in cabf55b

<Content Include="$(PackagesDir)\mlnetmkldeps\$(MlNetMklDepsPackageVersion)\LICENSE.txt" Pack="true" PackagePath=".\NOTICE.txt" />

[codecov]

Codecov Report

Merging #2285 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2285      +/-   ##
==========================================
+ Coverage   69.89%   69.91%   +0.01%     
==========================================
  Files         786      786              
  Lines      144268   144268              
  Branches    16635    16635              
==========================================
+ Hits       100842   100866      +24     
+ Misses      38876    38849      -27     
- Partials     4550     4553       +3

Flag	Coverage Δ
#Debug	69.91% <ø> (+0.01%)	⬆️
#production	66.18% <ø> (+0.01%)	⬆️
#test	85.01% <ø> (+0.01%)	⬆️