Fix typing for TS AnimatableStringValue (#36366)

Summary:
Pull Request resolved: #36366

## Changelog:

[General][Fixed] -

#36346 added some typing improvements, however there was a typo in `AnimatableStringValue` type definition, that broke tests on CI.

Reviewed By: cortinico, cipolleschi, hoxyq

Differential Revision: D43770412

fbshipit-source-id: 7b4f234b5cf04df9271e0c98cf51655c87e3bebb

Libraries/StyleSheet/StyleSheetTypes.d.ts

@@ -25,7 +25,7 @@ type DimensionValue =
   | Animated.AnimatedNode
   | null;
 type AnimatableNumericValue = number | Animated.AnimatedNode;
-type AnimatableStringValue = number | Animated.AnimatedNode;
+type AnimatableStringValue = string | Animated.AnimatedNode;
 
 /**
  * Flex Prop Types