Fix typo 'principal' -> 'principle' in ch02

[danny-lloyd]

(I haven't opened an issue for discussion of these changes because it is a small typo)

Not sure what needs doing with the checklist below as this is a very minor typo!

Release checklist

• [ ] Authors updated in cf-conventions.adoc? Add in two places: on line 3 and under .Additional Authors in About the authors.
• [ ] Next version in cf-conventions.adoc up to date? Versioning inspired by SemVer.
• [ ] history.adoc up to date?
• [ ] Conformance document up to date?

For maintainers

After the merge remember to delete the source branch. Tags are set at the conclusion of the annual meeting; until then, main always is a draft for the next version.

[JonathanGregory]

Thanks for spotting and correcting the spelling mistake, @danny-lloyd. I will merge it.