spec: uniformly format empty interfaces as "interface{}"

Fixes #36526.

Change-Id: Ic51a287579f139422cc1a7b2fb82d6732114b031
Reviewed-on: https://go-review.googlesource.com/c/go/+/214597Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/go_spec.html

 <!--{
 	"Title": "The Go Programming Language Specification",
-	"Subtitle": "Version of Dec 13, 2019",
+	"Subtitle": "Version of Jan 13, 2020",
 	"Path": "/ref/spec"
 }-->
 
@@ -5279,7 +5279,7 @@ for i, s := range a {
 }
 
 var key string
-var val interface {}  // element type of m is assignable to val
+var val interface{}  // element type of m is assignable to val
 m := map[string]int{"mon":0, "tue":1, "wed":2, "thu":3, "fri":4, "sat":5, "sun":6}
 for key, val = range m {
 	h(key, val)