fix(docs): update parameter name

[apogiatzis]

Minor update of the documentation when creating a GithubRepository block.

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[zzstoatzz]

hi @apogiatzis - thanks for the PR, but this is not correct - GitRepository is not the same as the GitHubRepository block defined in prefect-github. The correct parameter here is url, see https://github.com/prefecthq/prefect/blob/main/src/prefect/runner/storage.py#L83-L117

[apogiatzis]

Ah you are right! Apologies, I could have sworn that I'd copied the code from documentation and I encountered the parameter mismatch while testing but I guess I must have changed it to GithubRepository.