types: fix typo in CustomInstanceExtensions

[marcalexiei]

During #2121 I noticed a typo in CustomInstanceExtensions so I corrected it.
I don't know if this should be considered a breaking change or a bug but there is no mention in the doc of this interface (https://www.i18next.com/?q=CustomInstanceExtensions), hence I think it can be considered a fix.

Checklist

• only relevant code is changed (make a diff before you submit the PR)
• run tests npm run test
• (N/A) tests are included
• commit message and code follows the Developer's Certification of Origin

[coveralls]

coverage: 96.239%. remained the same
when pulling 6ad199b on marcalexiei:feature/fix-types-interface-typo
into 44e132d on i18next:master.

[adrai]

Only used here: https://github.com/i18next/react-i18next/blob/26f83a89b30c8b25a0d400d45d7d63525d5f9c28/index.d.ts#L40
so also react-i18next needs to be fixed.
currently afk... will check in a couple of hours

[marcalexiei]

Gotcha, I'll open also a PR there.

[adrai]

thanks a lot