docs(MigrationGuide): correct spacing-variable name (#6588)

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## Pull Request Checklist
- [x] Reviewed the [Contributing
Guidelines](https://github.com/SAP/ui5-webcomponents-react/blob/main/CONTRIBUTING.md)
+ Especially the [How to
Contribute](https://github.com/SAP/ui5-webcomponents-react/blob/main/CONTRIBUTING.md#contribute-code)
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Author: mrCherry97

docs/MigrationGuide.mdx

@@ -162,7 +162,7 @@ Most variables can be replaced by applying the corresponding CSS classes from th
 | `sapUiLargeMarginBeginEnd`          | `.sap-margin-x-large`         |
 | **Vertical Margins**                |                               |
 | `sapUiTinyMarginTopBottom`          | `.sap-margin-y-tiny`          |
-| `sapUiSmallMarginBeginEnd`          | `.sap-margin-y-small`         |
+| `sapUiSmallMarginTopBottom`         | `.sap-margin-y-small`         |
 | `sapUiMediumMarginTopBottom`        | `.sap-margin-y-medium`        |
 | `sapUiLargeMarginTopBottom`         | `.sap-margin-y-large`         |
 | **Responsive Margins**              |                               |