Use `client` instead of `impresso` in documentation

[theorm]

As ChatGPT puts it:

Shadowing the module name like
import impresso
impresso = impresso.connect()
technically works, but it’s a bad idea.
• You lose access to the impresso module (no impresso.DateRange, no help(impresso) on the module).
• It breaks introspection and autocomplete.
• It can cause subtle bugs, especially in dynamic/reloading environments.
• It’s confusing for anyone reading or maintaining the code.
✅ Better:
import impresso
client = impresso.connect()
Keeps everything accessible and clear. (edited)

[caiocmello]

I made this change in all datalab notebooks already.