Fix latex subscript, etc use in ode.py lines 3671-2

Fixes #7417
Made some edits of latex and edited desription of ode_1st_power_series doc

More Fixes for #7417
Whitespace removed in ode.py
Fix too long line.

On line 2782 in ode.py there was a too long line. I separated the line into two
Fixed the subscripting for x_{0}.
Fixed need for :math: in 5386
Removed mention of accidentally included file.
changed x0 to x_{0}
Merge pull request #7530 from comer/Fix_LaTeX_ODE_doc_for_7417

Fix la te x ode doc for 7417
Removed stray print statement
Merge branch 'master' of https://github.com/sympy/sympy into basicvector