docs: fix response.vary jsdoc field param

[hustLer2k]

Vary can accept either a single string or an array of strings, as per vary's docs and types

Checklist

• ✔️ I have ensured my pull request is not behind the main or master branch of the original repository.
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[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (dbf4b8f) 98.66% compared to head (ad45f77) 98.66%.

:exclamation: Current head ad45f77 differs from pull request most recent head 3e60267. Consider uploading reports for the commit 3e60267 to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1767 +/- ## ======================================= Coverage 98.66% 98.66% ======================================= Files 5 5 Lines 523 523 Branches 146 146 ======================================= Hits 516 516 Misses 7 7 ``` | [Impacted Files](https://app.codecov.io/gh/koajs/koa/pull/1767?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=koajs) | Coverage Δ | | |---|---|---| | [lib/response.js](https://app.codecov.io/gh/koajs/koa/pull/1767?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=koajs#diff-bGliL3Jlc3BvbnNlLmpz) | `100.00% <ø> (ø)` | |

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[hustLer2k]

@tj @jonathanong @dead-horse @fengmk2 @tejasmanohar