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/+/165917Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/effective_go.html

@@ -1687,6 +1687,8 @@ map[CST:-21600 EST:-18000 MST:-25200 PST:-28800 UTC:0]
 </pre>
 <p>
 For maps, <code>Printf</code> and friends sort the output lexicographically by key.
+</p>
+<p>
 When printing a struct, the modified format <code>%+v</code> annotates the
 fields of the structure with their names, and for any value the alternate
 format <code>%#v</code> prints the value in full Go syntax.