update write callback to use const

Co-authored-by: Fabian Sauter <sauter.fabian@mailbox.org>

Merge pull request #36 from RuleOfThrees/multiasync_doc

Documentation for MultiAsync

Merge pull request #27 from leviliangtw/bugfix/issue769

Update documentation for retrieving certificate information of cpr::Response

Added docs for reserving space for response strings

Added docs for reserving space for response strings

Added docs for reserving space for response strings

Added docs for reserving space for response strings

Added docs for reserving space for response strings

Fixed the "Download With Callback" example

Bump github-pages from 221 to 222

Bumps [github-pages](https://github.com/github/pages-gem) from 221 to 222.
- [Release notes](https://github.com/github/pages-gem/releases)
- [Commits](github/pages-gem@v221...v222)

---
updated-dependencies:
- dependency-name: github-pages
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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