[ISSUE #195] fix a typo in readme

[usernameisnull]

What is the purpose of the change

fix a typo in readme.md

Brief changelog

Iamge -> Image

Verifying this change

just see it
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• Make sure there is a Github issue filed for the change (usually before you start working on it). Trivial changes like typos do not require a Github issue. Your pull request should address just this issue, without pulling in other changes - one PR resolves one issue.
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