Fix grammar in Doc/tutorial/controlflow.rst

[infrastation]

Automerge-Triggered-By: @csabella

[the-knights-who-say-ni]

Hello, and thanks for your contribution!

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We couldn't find a bugs.python.org (b.p.o) account corresponding to the following GitHub usernames:

@infrastation

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[infrastation]

Updated the BPO user account and submitted the CLA.

[tirkarthi]

LGTM. Thanks.

[miss-islington]

Sorry, I can't merge this PR. Reason: Required status check "Travis CI - Pull Request" is expected..

[csabella]

Closing to re-trigger Travis CI.

[miss-islington]

@infrastation: Status check is done, and it's a success ✅ .

[miss-islington]

@infrastation: Status check is done, and it's a success ✅ .

[miss-islington]

Sorry, I can't merge this PR. Reason: Required status check "Travis CI - Pull Request" is expected..

[miss-islington]

Sorry, I can't merge this PR. Reason: Required status check "Travis CI - Pull Request" is expected..

[miss-islington]

@infrastation: Status check is done, and it's a success ✅ .

[miss-islington]

Thanks @infrastation for the PR 🌮🎉.. I'm working now to backport this PR to: 3.8, 3.9.
🐍🍒⛏🤖

[bedevere-bot]

GH-21922 is a backport of this pull request to the 3.9 branch.

[bedevere-bot]

GH-21923 is a backport of this pull request to the 3.8 branch.

[tirkarthi]

Thanks @infrastation for your contribution :) Thanks @csabella for taking care of this :)