docs/index: use a relative link to javadoc pages

The absolute URL breaks API doc versioning, since all versions are
pointing to the same URL. With the relative link in place, browser
will include the version in the request URL.

Base URL:
  http://apidocs.strongloop.com/loopback-sdk-android/v/1.4.1/

Relative link:
  api/index.html

Result:
  http://apidocs.strongloop.com/loopback-sdk-android/v/1.4.1/api/index.html