Add "merge" label for `pull` Network diagram flow

Fixes missing `merge` label on `pull` step of network diagram. Addresses https://github.com/github/training.github.com/issues/165.
![screen shot 2013-12-18 at 11 33 45 am](https://f.cloud.github.com/assets/352082/1776182/fea1b96e-6812-11e3-9b94-a136cdca43c5.png)