[heycam/webidl] Fix link to ES spec function object (#410)

This currently reads:

```
text: function object; url: sec-object-internal-methods-and-internal-slots
```

Once tc39/ecma262#974 lands, this should be changed to:

```
text: function object; url: function-object
```

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https://github.com/heycam/webidl/issues/410