chore: update readme to use .by instead of group_by()

[wurli]

Please feel free to reject this if you prefer to keep as-is. Just thought the upcoming release might be a nice time to make this change since .by is no longer experimental :)

Thanks for the great work on the latest release!

[DavisVaughan]

This is slightly inconsistent with the opening paragraph

These all combine naturally with `group_by()` which allows you to
perform any operation “by group”. You can learn more about them in
`vignette("dplyr")`. As well as these single-table verbs, dplyr also
provides a variety of two-table verbs, which you can learn about in
`vignette("two-table")`.

Since then we use a different grouping style in the examples. But i think it just requires a little tweaking. I'll take a look, I would not be opposed to this