doc: use appropriate type to describe return value

Fixes #19223.

Change-Id: I4cc8e81559a1313e1477ee36902e1b653155a888
Reviewed-on: https://go-review.googlesource.com/37374Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

doc/effective_go.html

@@ -1580,7 +1580,7 @@ if attended[person] { // will be false if person is not in the map
 <p>
 Sometimes you need to distinguish a missing entry from
 a zero value.  Is there an entry for <code>"UTC"</code>
-or is that the empty string because it's not in the map at all?
+or is that 0 because it's not in the map at all?
 You can discriminate with a form of multiple assignment.
 </p>
 <pre>