Small fixes to Euler chapter

[leios]

Bug Report

Description

Notational error in Taylor expansion for Euler method and images are incorrectly labelled

Note, there was a private communication on discord about this.

For Algorithm Archive Developers

• [ ] The bug can be reproduced
• [ ] The bug can be fixed (if not, please explain why not in a comment below)
• [ ] There is a timeline to fix the bug
• [ ] The bug has been fixed (Please link the PR)

[leios]

Lots of issues with the Euler chapter. I am working on them. One note: I feel like the code should be using function pointers or something rather than directly embedding the ODE into the solve_euler() function.

I know I can fix this when talking about RK methods, but maybe we should also fix the code?