Fix backslash not working on non-windows machine

[EltonGohJH]

What is the purpose of this pull request?

• Documentation update
• Bug fix
• Feature addition or enhancement
• Code maintenance
• DevOps
• Improve developer experience
• Others, please explain:

Overview of changes:
Backslash does not work on non-windows machine.
To fix it I replace backslash with path.sep.
Fixes #2161

Anything you'd like to highlight/discuss:

Testing instructions:
Can try running on CS2103T site with non-windows machine.
Do note that there is some bug with CS2103T with non-windows machine due to naming.
I have created a PR there. (nus-cs2103-AY2223S2/website#1)
So for now, you might encounter some error message when serving cs2103t site on non-windows machine.

Proposed commit message: (wrap lines at 72 characters)

Fix backslash not working on non-windows machine

Currently, backslash is not supported on Markbind. So, non-windows
user will encounter error when including file with backslash.

We can fix the changing backslash to forwardslash of the include path.

This will allow non-windows user to be able to serve website on Markbind 
when they use forwardslash.

This change resolves the following issue:
* #2161: Back slash not working for include path when using Markbind 4+

---

Checklist: ☑️

• Updated the documentation for feature additions and enhancements
• Added tests for bug fixes or features
• Linked all related issues
• No unrelated changes

[lhw-1]

HI @EltonGohJH, thanks for the catch! LGTM :)