Broken link to W3Schools

I see the external link to XPATH-tutorial on W3Schools has moved from 
`http://www.w3schools.com/xsl/xpath_intro.asp` to 
`http://www.w3schools.com/xml/xpath_intro.asp`
Patch [submitted](https://github.com/kennethreitz/python-guide/compare/master...vharsh:patch-1) 