docs/contributing.md: remove unnecessary quote

[rogeryk]

There is an extra quote in the command, which should be an error.

[gopherbot]

This PR (HEAD: 703b7a1) has been imported to Gerrit for code review.

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[gopherbot]

Message from qiulaidongfeng:

Patch Set 1: Run-TryBot+1

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[gopherbot]

Message from Hyang-Ah Hana Kim:

Patch Set 1: Code-Review+2

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[gopherbot]

Message from Robert Findley:

Patch Set 1: Code-Review+2

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[gopherbot]

Message from Hyang-Ah Hana Kim:

Patch Set 2: Auto-Submit+1 Code-Review+2 Commit-Queue+1 Run-TryBot+1

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After addressing review feedback, remember to publish your drafts!

[gopherbot]

Message from kokoro:

Patch Set 2:

Kokoro presubmit build queued for golang/vscode-go/gcp_ubuntu/presubmit
Logs at: http://sponge2.corp.google.com/ecd3c702-7cef-4075-986b-2aad257a52d3

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[gopherbot]

This PR is being closed because golang.org/cl/541875 has been merged.