Fix la te x ode doc for 7417

I am submitting a PR for fixes to #7417.  I have cleaned up the latex wherever I could see problems. I also have edited the doc for the sympy.solvers.ode.ode_1st_power_series function. Its description was not very coherent.

Note that there may well be other latex items I either did not see or did not fix, but I think now the latex is cleaner.
This should also be `x_{0}`