html/atom: add atom.Acronym

Fixes golang/go#23507

Change-Id: Id39b28f211dfdb6a5464752b8b62e2528b46286b
Reviewed-on: https://go-review.googlesource.com/91315Reviewed-by: David Symonds <dsymonds@golang.org>

html/atom/gen.go

@@ -665,6 +665,7 @@ var eventHandlers = []string{
 
 // extra are ad-hoc values not covered by any of the lists above.
 var extra = []string{
+	"acronym",
 	"align",
 	"annotation",
 	"annotation-xml",

html/atom/table_test.go

@@ -10,6 +10,7 @@ var testAtomList = []string{
 	"accept",
 	"accept-charset",
 	"accesskey",
+	"acronym",
 	"action",
 	"address",
 	"align",