Add remark about path separator to use in Unix (#3680)

* Add remark about path separator to use in Unix

If you are creating directories on a Unix system using .NET Core the path separator must a forward slash character.
I think this remark is missing. All the examples use Windows path separators (\\).

* Update xml/System.IO/Directory.xml

Co-Authored-By: Maira Wenzel <[email protected]>

Co-authored-by: Maira Wenzel <[email protected]>

Author: boskjoett

xml/System.IO/Directory.xml

@@ -173,7 +173,7 @@
   
  Creating a directory with only the colon character (:) is not supported, and will cause a `NotSupportedException` to be thrown.  
   
-   
+ On Unix systems, use a forward slash (/) as path separator.  
   
 ## Examples