[heycam/webidl] Editorial: fix language in nullable types section (#628)

This PR fixes the following sentence:

```diff
- The inner type must not be […] a union type that itself has includes a nullable type 
+ The inner type must not be […] a union type that itself includes a nullable type 
```

cf. section https://heycam.github.io/webidl/#idl-nullable-type

You can view, comment on, or merge this pull request online at:

  https://github.com/heycam/webidl/pull/628


-- Commit Summary --

  * Editorial: fix language in nullable types section

-- File Changes --

    M index.bs (2)

-- Patch Links --

https://github.com/heycam/webidl/pull/628.patch

https://github.com/heycam/webidl/pull/628.diff


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