Change sourceDir to src/ in generate-info.js

This is to accomodate the case where other directories than "ol" include "api" annotations. For example, the Swisstopo geoadmin folks extend OpenLayers 3 in an ol3 fork, and they have their own "ga" namespace/directory under "src". See https://github.com/geoadmin/ol3/tree/master/src.
Merge pull request #2174 from elemoine/generate-info

Change sourceDir to src/ in generate-info.js