docs: Correct the definition of an expression in the user guide

[MarcoGorelli]

I hope it's OK to suggest this change - it's been bugging me for a bit, and trying to translate the Polars API to the pandas API I kept coming up against this

If we consider the expression expr = pl.sum_horizontal('a', 'b').abs(), then it roughly translates to

def expr(df):
   return (df['a'] + df['b']).abs()

So the input is a dataframe, not a series. The output is always a Series, which is why you can keep chaining expressions - they just keep acting on the output Series from the previous step

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EDIT: I think I also heard Orson describe expressions in these terms, but can't find the message now

[stinodego]

Looks like a good adjustment, thanks!