WEB/CI: Fixing target path of the web build
http://dev,pandas.io is currently not serving the website (http://dev,pandas.io/docs)

In the deployment of the new website, when building the web, the target default path `build` is not working as expected. The path is later expected to be `web/build`, but since the script is called from the root and not from inside web, the target path is `build/`, and the website is not copied.

This PR fixes the target path to be the correct one, and should fix http://dev,pandas.io

@TomAugspurger if you don't mind having a quick look.
Thanks Tom, all working now.