fixed template of "index-all.html" to be parsed by Dash's javadocset command

If you are mac user, Dash is very handy tool to look up API document.
`index-all.html` of groovy-jdk from current template can't be parsed by Dash's `javadocset` command.

After this patch is applied, you can get the groovy-jdk's docset as follows:

```
$ cd groovy-core
$ gradle docGDK
$ javadocset "Groovy JDK" target/html/groovy-jdk
$ ls "Groovy JDK.docset"
Groovy JDK.docset
```


* Dash - http://kapeli.com/dash
* javadocset - https://github.com/Kapeli/javadocset