Clarify final expression return wording in ch09-02

[NMJ27]

The current wording says “this is the last expression in the function,” which may be read as referring to Err(e) in the Err arm rather than to the entire match expression. This change makes that more explicit for learners.

It rewords the sentence to:

However, we don’t need to explicitly say return before the final match, because the entire match expression is the last expression in the function, so its value is returned.

When reading this section, I initially interpreted the earlier Err(e) => return Err(e) as a pattern that also applied to the final match. In that reading, the omitted return can seem unclear, especially for the Ok(username) arm. This change clarifies that the omitted idiomatic return applies to the entire final match, not to an individual arm.