Fix a typo in Feature Flag rollout template

As title implies, this MR is to fix a typo in the Feature Flag rollout template.

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Conformity

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• Documentation (if required)
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• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

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• Label as security and @ mention @gitlab-com/gl-security/appsec
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