Typo fix Update contracts.md

[Hack666r]

Fix Typo in contracts.md

This pull request addresses a minor typo in the contracts.md file. Specifically:

• Corrected requestor to requester for consistency and accuracy.

No functional changes were made, only a documentation improvement for better clarity.

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Changes:

- The requestor can be the account itself or an app that wants to operate on its behalf.
+ The requester can be the account itself or an app that wants to operate on its behalf.


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## PR-Codex overview
This PR focuses on correcting a typographical error in the documentation regarding the `Key Registry`. The term "requestor" has been changed to "requester" for consistency and accuracy.

### Detailed summary
- Changed "requestor" to "requester" in the description of the `Key Registry` in `docs/learn/architecture/contracts.md`.

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