doc: add missing paragraph break in Effective Go

A recent edit broke the flow; add a paragraph break when the subject
switches from maps to structs.

No changes in wording.

Change-Id: I5df88ec36b9d81931cfdbc684424440d01ac06d1
Reviewed-on: https://go-review.googlesource.com/c/go/+/165917


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>