chore(readme): use lower-case header name

[SchroederSteffen]

Fix #859, see #859 (comment).

HTTP headers are supposed to be case-insensitive and superagent lower-cases them to unify the case.
The example in the README doesn't work, because it uses Content-Type instead of content-type. This PR fixes the example.

Checklist

• I have ensured my pull request is not behind the main or master branch of the original repository.
• I have rebased all commits where necessary so that reviewing this pull request can be done without having to merge it first.
• I have written a commit message that passes commitlint linting.
• I have ensured that my code changes pass linting tests.
• I have ensured that my code changes pass unit tests.
• I have described my pull request and the reasons for code changes along with context if necessary.

[A1exKH]

LGTM.

[titanism]

v7.2.0 published to npm

https://github.com/forwardemail/supertest/releases/tag/v7.2.0

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