Github feed example shows "function" as valid key for Feed

Github feed example shows this as valid:

```
git_webhook:
version: 1.0
license: Apache-2.0
feeds:
webhook_feed:
version: 1.0
function: github/webhook.js
runtime: nodejs@6
```

however, the key "**function**" is not supported in the actual spec. Perhaps this is an attempt at some shortcut that we have not discussed/vetted properly?

The only way to ref a function is indirectly, using an "**action**" (i.e. the name of an action also declared in the manifest file.