Update finalsize_comparison.Rmd

[adamkucharski]

The vignette comparing finalsize and epidemics is currently framed on numerical comparison, which adds limited value for users (because two equivalent models should produce very similar numerical outputs). Instead, have tweaked to frame around input complexity, as users of epidemics may define a complex model but not have all the parameters defined, so this vignette is now framed around how they can use finalsize in meantime in an integrated way (and the advantages/disadvantages of taking these approaches).
epiverse-trace/finalsize#190

• Please check if the PR fulfills these requirements

• [ X] I have read the CONTRIBUTING guidelines
• A new item has been added to NEWS.md - Not included as this PR is only small copy changes in documentation rather than any functionality/code changes.
• [ X] Tests for the changes have been added (for bug fixes / features)
• [ X] Docs have been added / updated (for bug fixes / features)

[pratikunterwegs]

Thanks @adamkucharski, happy to go with your additions.