chore(docs): Fix grammar in add-404-page

[nathanchu]

Description

I've fixed the phrasing in the add-404-page.md document:

- To create a 404 page create a page whose path matches the regex `^\/?404\/?$` (`/404/`, `/404`, `404/` or `404`). Most often you'll want to create a React component page at `src/pages/404.js`.
+ To create a 404 page create a page whose path matches the regex `^\/?404\/?$` (`/404/`, `/404`, `404/` or `404`), most often you'll want to create a React component page at `src/pages/404.js`.

Documentation

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Related Issues

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This is my first PR to gatsby, if there's anything I should do please tell me.

[LekoArts]

Thanks!