Links to upper directories fail in md files

[marcindulak]

The example below uses docker #23

1. build the image of the latest linkspector from source

   git clone https://github.com/UmbrellaDocs/linkspector
   cd linkspector
   docker build --build-arg LINKSPECTOR_PACKAGE= -t umbrelladocs/linkspector .
   

2. run linkspector check on a test directory

   mkdir -p test/nested
   cp .gitignore test  # due to https://github.com/UmbrellaDocs/linkspector/issues/24
   echo '[test-relative](../test.md)' > test/nested/nested.md
   echo '[nested-absolute](/nested/nested.md)' > test/test.md
   echo '[nested-relative](./nested/nested.md)' >> test/test.md
   docker run --rm -it -v $PWD/test:/app --name linkspector umbrelladocs/linkspector bash -c 'linkspector check'
   

Output

⠋ Configuration file not found. Using default configuration.
🚫 nested/nested.md, ../test.md, 404, 1, Cannot find: ../test.md.
❌ Error: Some links in the specified files are not valid.

It is expected that 'linkspector check' passes. It is a reproduction example for #22