Update Path.xml documentation (#4252)

Unix doesn't use \ but /. Running this on OSX gives `/`

Author: jcolebrand

xml/System.IO/Path.xml

@@ -1643,7 +1643,7 @@ Possible patterns for the string returned by this method are as follows:
 
 - An empty string (`path` specified a relative path on the current drive or volume).
 
-- "\" (Unix: `path` specified an absolute path on the current drive).
+- "/" (Unix: `path` specified an absolute path on the current drive).
 
 - "X:" (Windows: `path` specified a relative path on a drive, where *X* represents a drive or volume letter).