Fix typo in README.md

[tvogels]

I fixed a small typo in inception/slim/README.md.

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[tvogels]

I signed it!

[googlebot]

CLAs look good, thanks!

[girving]

@tvogels: Can you rebase on top of master? Github seems to be complaining about conflicts.

@martinwicke: What's the testing / merging story for the models repo?

[tvogels]

Okay, done.

[martinwicke]

Thank you!