The `--expect DIR` switch requires a trailing slash

[Kakadu]

I was surprised that --expect switch requires a directory with slash to properly check that it is in report. Without a lash it complains. Maybe we can insert this trailing slash automatically? @aantron

✗ bisect-ppx-report html --coverage-path /tmp/zanudacov  --expect src 
Error: expected file 'src' is not included in the report
✗ bisect-ppx-report html --coverage-path /tmp/zanudacov  --expect src/
✗ # OK