TVB-2772: Corrected equations in Phase Plane

Created by: VinczeRobert

A fixed the obvious syntax errors and I removed the commas from the end of the equations (I didn't get the meaning of them, somewhere they where put, somewhere they weren't, I don't think they look good but I can put them back if it's needed for some reason). A few things I would like to point out:

1. The Linear and the Zetterberg-Jansen function don't have a documentation in their dfun function, I thought to add them myself from the class doc, cause I noticed that usually the dfun doc is like a shorter version of the class doc. I didn't add them yet, but I could.
2. I think I should still look in the mathjax syntax of the model class variables cause there are some problems there too.
3. I think someone who has expertise in this area should look if the equations are correct. I think there might be errors, I even found one possible mistake myself. Look at the third equation of the Reduced Wong-Wang With Excitatory.... model. It is can be found here https://www.researchgate.net/publication/245029745_Resting-State_Functional_Connectivity_Emerges_from_Structurally_and_Dynamically_Shaped_Slow_Linear_Fluctuations (equation 6). If you look there, there is an additional term in the end which we don't have.