Update index.Rmd

[wdecoster]

Thanks for the great resource. My understanding of statistics is poor, so this document is really helpful. What would help me a lot is if every test would have a simple one-sentence example of what kind of question is answered here. Perhaps all using the same dataset, e.g. is the sepal width of iris setosa significantly different from the sepal width of versicolor...

This PR fixes some typos I encountered while reading through.

I think something might be wrong with the p-value of the lm test in 6.1.3 R code: one-way ANOVA. It's a lot off from the Anova p-value, and in the R-output below I think p-value: 0.009984 is what should replace the 0.73795. I'm not too sure about this so I didn't include it in the PR.