Fix a bug in the template serving.
Fix a bug in the template serving.

PR #880 introduced a bug in the template rendering for the notebook
UI where the name of the template was wrong (`notebook` instead of
`nb`).

This change fixes that.

googledatalab/datalab,parthea/datalab,parthea/datalab,yebrahim/datalab,googledatalab/datalab,parthea/datalab,googledatalab/datalab,yebrahim/datalab,craigcitro/datalab,googledatalab/datalab,googledatalab/datalab,yebrahim/datalab,yebrahim/datalab,craigcitro/datalab,parthea/datalab,craigcitro/datalab,parthea/datalab,yebrahim/datalab,craigcitro/datalab,craigcitro/datalab,parthea/datalab,googledatalab/datalab