[DOCS] Fix typos in joins.md

[emdede]

I just went through this document a few days ago and found it a bit confusing being new to drizzle. I went ahead and cleared up the confusing parts, which seem to be mostly code relics from earlier versions.

Replaced fields() with select()
Clarified key naming without select() argument
Aggregation example was incorrectly using cityId

[dankochetov]

Good catch, thanks!