Fix flow type for maxFontSizeMultiplier on TextInput (#21271)

Summary:
This flow type is wrong, probably just a copy paste mistake.
Pull Request resolved: https://github.com/facebook/react-native/pull/21271

Differential Revision: D10006741

Pulled By: TheSavior

fbshipit-source-id: eba0116ec39ba00f000d9bf789ae9214990355a1

Libraries/Components/TextInput/TextInput.js

@@ -179,7 +179,7 @@ type Props = $ReadOnly<{|
   autoCorrect?: ?boolean,
   autoFocus?: ?boolean,
   allowFontScaling?: ?boolean,
-  maxFontSizeMultiplier?: ?boolean,
+  maxFontSizeMultiplier?: ?number,
   editable?: ?boolean,
   keyboardType?: ?KeyboardType,
   returnKeyType?: ?ReturnKeyType,