add call to endGenerateDocumentation at end of docs generation

the method `endGenerateDocumentation` existed but was not being called, the result being (at least, the result for me, perhaps it affects other usages too) that when grunt-docular uses docular, the callback never gets run and so the grunt build effectively 'exits' after running the grunt-docular task.

i raised this as an issue in the grunt-docular project:

https://github.com/gitsome/grunt-docular/issues/6

before i tracked it down to here.