Fix class declaration keyword in liquid tags and helpers docs

[ayushn21]

This is a 🔦 documentation change.

Summary

Documentation fixes on the plugins/tags and plugins/helpers pages where it seems a class was declared with def. I've just replaced def with class.

This PR fixes #197

[jaredcwhite]

Thanks @ayushn21! Do you mind addressing #197 as well and we can fix it all together? (There must have been a typo in an initial example and then it propagated around other parts of the helpers/filters/tags docs… 😬)

[ayushn21]

Yup sure @jaredcwhite; will add those fixes in as well :)

[ayushn21]

I've just pushed the fixes for the helpers page as well :)

[jaredcwhite]

@ayushn21 Niiiice 👍