Fix alt text for Feature Flag Metrics

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Related to: https://gitlab.com/gitlab-org/gitlab/-/issues/479161

This MR is related to the open issue: #479161 titled Update alt text for images

As per given guidelines, the spreadsheet given in the issue link has been updated.
doc/development/feature_flags/controls.md [Row 272]

Required update for the alt text has been completed describing the metric illustration with Apdex score variation with the feature flag changes at given times.
The styling guideline given here here has been followed for the change.