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.

Discussed in https://github.com/openlayers/ol3/commit/f9157a61231a230df07ed09792cecaa39f57164f#commitcomment-6582726.

CC @gjn.