html/template: fix example code

Fixes #9651

Change-Id: I987833b6263482a402e58fcd9eeb0e42401599b5
Reviewed-on: https://go-review.googlesource.com/3073Reviewed-by: Robert Griesemer <gri@golang.org>

src/html/template/doc.go

@@ -151,7 +151,7 @@ The template
 
 can be invoked with
 
-  tmpl.Execute(out, HTML(`<b>World</b>`))
+  tmpl.Execute(out, template.HTML(`<b>World</b>`))
 
 to produce