Fix the quickstart documentation

[sb2nov]

R: @aaltay PTAL

[asfbot]

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Jenkins built the site at commit id bd158b5 with Jekyll and staged it here. Happy reviewing.

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[aaltay]

LGTM. We should eventually have tabs here for DirectRunner and DataflowRunner separately.

[aaltay]

https://issues.apache.org/jira/browse/BEAM-1549 for updating the quickstart with tabs.

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