doc: isabsolute() windows examples use wrong path separator

[rtpHarry]

Doc incorrectly uses forward slashes for path examples on Windows platform:

https://nodejs.org/docs/latest/api/path.html#path_path_isabsolute_path

path.isAbsolute('//server')  // true
path.isAbsolute('C:/foo/..') // true

Should be

path.isAbsolute('\\server')  // true
path.isAbsolute('C:\foo\..') // true

[cjihrig]

It might be worth noting in the docs that forward slashes work too. The example could also add an absolute path using backslashes to go along with what is already there.

Care to make a PR?