Fix example in OS.shell_open() method documentation and add a new one

[mateuseap]

Closes #10367

What I Did:

• Fixed an issue with string escaping in an example provided for the OS.shell_open() method documentation. Now the example is properly escaped to reflect valid Windows path syntax.
• Added a new example of how to open a Windows path using the OS.shell_open() method with a different syntax from the first Windows path example.

Demonstrations:

• Current Windows path example shown in the OS.shell_open() documentation displaying error:
  
• Corrected Windows path example showing no errors:
  
  • Note: I've tested this GDScript, and it opens the folder as expected. I attached the script to a node and ran the scene, and it worked perfectly. If needed, I can provide a GIF.

[bruvzg]

Note: shell_open on Windows will also work with /, so [code]OS.shell_open("C:/Users/name/Downloads")[/code] can be added as alternative example as well.

[Mickeon]

I think we could just stick to using / but I have no strong opinion on it. Windows is the odd one out.

[mateuseap]

@bruvzg I've added a new example using / only, as you suggested!

[akien-mga]

Thanks!