Fix typo in text_to_image/README.md

[lunbun]

I believe that --outdirname= was mistakenly written as --outdirname-.

[github-actions]

MLCommons CLA bot:
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[arjunsuresh]

Can you please sign the CLA?

[lunbun]

recheck

[lunbun]

Hi, would it be okay to close this PR, instead open an issue about this, and someone else make a commit? I would prefer to not sign the CLA for privacy reasons, especially since this is such a small change.