Fixed typo in modelzoo tutorial

[bisognano]

Was running through the tutorials and this caught my eye. Sorry if its too small an issue!

[review-notebook-app]

Check out this pull request on ReviewNB: https://app.reviewnb.com/tensorflow/lucid/pull/158

Visit www.reviewnb.com to know how we simplify your Jupyter Notebook workflows.

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[bisognano]

Signed the CLA @googlebot

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CLAs look good, thanks!

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[coveralls]

Pull Request Test Coverage Report for Build 493

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 77.725%

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Totals
Change from base Build 492:	0.0%
Covered Lines:	1640
Relevant Lines:	2110

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💛 - Coveralls

[ludwigschubert]

@johnbisognano thanks!