missing parentheses in example code

Summary:
in the example code for configuring animations, Easing.back should be Easing.back()

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Closes https://github.com/facebook/react-native/pull/15273

Differential Revision: D5929874

Pulled By: shergin

fbshipit-source-id: 89921d0dda899e1697ee1fa5d96752964a2dc3f9

docs/Animations.md

@@ -106,7 +106,7 @@ Animated.timing(
   this.state.xPosition,
   {
     toValue: 100,
-    easing: Easing.back,
+    easing: Easing.back(),
     duration: 2000,
   }                              
 ).start();