html/template: Add missing error check to package example.

This appears to be an unintended omission. The check func is declared
just above, and the err value from template.Parse is captured rather
than discarded via blank identifier. All following calls that similarly
return err are checked, so it can't be that this example elides error
checking for brevity. Finally, if you look at Example_autoescaping,
it does check err from template.Parse and its code is very similar.

Change-Id: I076e1846302d5f2cdb1d027ed85ca0db85e33ace
Reviewed-on: https://go-review.googlesource.com/17170Reviewed-by: Andrew Gerrand <adg@golang.org>

src/html/template/example_test.go

@@ -31,6 +31,7 @@ func Example() {
 		}
 	}
 	t, err := template.New("webpage").Parse(tpl)
+	check(err)
 
 	data := struct {
 		Title string