Update README.md -- Ubuntu instructions reference

[jeremydouglass]

The Ubuntu instructions reference MacOS (a cut-paste error). Correct the typo.

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[jeremydouglass]

@googlebot I signed it!

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[bluesomewhere]

@jeremydouglass thank you so much for the PR! I'm the original author but no longer with Google, so don't presently have permissions to merge this in. Lemme see if I can bring this to someone's attention--or get myself re-added to the repo :)

[jeremydouglass]

@joshsimmons -- looks like another person has also submitted the same PR as #5. Maybe ping someone one more time?

[jeremydouglass]

Ping -- can anyone merge this simple typo fix?