docs/appengine: replace the "repeat" to "range"
I found repeat is not working in GAE now. 500 errors were returned:
```
panic: template: handler:2: function "repeat" not defined
goroutine 1 [running]:
panic(0x1889980, 0xc0105813c0)
go/src/runtime/panic.go:481 +0x3e6
html/template.Must(0x0, 0x2b4c416f1028, 0xc0105813c0, 0x0)
go/src/html/template/template.go:340 +0x4b
main43625.init()
hello.go:62 +0xcd
main.init()
_go_main.go:34 +0x59
```
@@ -70,7 +70,7 @@ func worker(w http.ResponseWriter, r *http.Request) {
var handlerTemplate = template.Must(template.New("handler").Parse(handlerHTML))
const handlerHTML = `
-{{repeat .}}
+{{range .}}
<p>{{.Name}}: {{.Count}}</p>
{{end}}
<p>Start a new counter:</p>
