Fix `conversionRate` PropType of `GasModalPageContainer` (#9651)

The `conversionRate` prop of `GasModalPageContainer` was updated
recently in PR #9623 to have a PropType of `string` instead of
`number`. This resulted in a PropType error whenever this modal was
rendered, as `conversionRate` is always a `number`.

The PropType has been reverted to the correct type, `number`.

ui/app/components/app/gas-customization/gas-modal-page-container/gas-modal-page-container.component.js

@@ -53,7 +53,7 @@ export default class GasModalPageContainer extends Component {
     customTotalSupplement: PropTypes.string,
     isSwap: PropTypes.bool,
     value: PropTypes.string,
-    conversionRate: PropTypes.string,
+    conversionRate: PropTypes.number,
     minimumGasLimit: PropTypes.number.isRequired,
   }