added loading of ggplot2 and manipulate libraries for first example

First example in 06_StatisticalInference/08_tCIs/index.Rmd will not work without loading ggplot2 and manipulate libraries so I added that.
It says 5 files changed, I don't know why. Hopefully someone can either merge or add this so I can sync to the master again.

Also, any lessons on why I can't remove previous pull requests (the other 4 files) and just submit this single one when I have already synced upstream with the master and no longer have those differences would be greatly appreciated