Double "O" section and misplaced and misformatted "Natural history of disease" in Glossary

[joshwlambert]

In the Glossary there are 2 issues spotted:

1. Natural history of disease seems to be in the wrong section ("O" instead of "N") and it's header is not being formatted correctly.
2. There are two "O" sections, the first with "Ordinary differential equations" and the second with "Offspring distribution".