Fix storybook relativePath depending on used platform

[Kuboczoch]

Description

Fix relativePath variable in storybook depending on the platform.
For my case using windows, I now can run yarn storybook without any issue

Fixes #40

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

I executed yarn storybook command from package.json

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I have checked my code and corrected any misspellings

[aeksco]

@Kuboczoch Very nice catch, thank you so much! Really appreciate the contribution 🥇