JOSS review: minor wording/formatting fixes to paper.md

[jbcaillau]

Editorial comment from the JOSS review requests a handful of minor wording and formatting fixes to joss/paper.md (and its bibliography). Tracking these here per JOSS convention.

Refers to openjournals/joss-reviews#10630, comment: openjournals/joss-reviews#10630 (comment)

Requested changes

• Title: replace "ODE's" with "ODEs".
• GPOPS-II description: "GPOPS-II delivers mature methods with MATLAB and C++ implementations" is misleading — GPOPS-II is a MATLAB-only package; make the description more precise.
• Shooting method convergence claim: "converges to arbitrary precision" is too strong for a numerical method — replace with a qualified statement (convergence to a prescribed numerical tolerance).
• Direct vs. indirect methods characterization: "direct methods for constrained problems with many variables, indirect methods for theoretical analysis and smaller problems requiring high accuracy" reads as categorical — reword as typical use cases rather than general limitations.
• Bibliography capitalization: protect capitalization-sensitive proper names, especially "Julia", so they aren't rendered in lowercase in the reference list.