typo in the error message (missing a space due to newline)

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Closes https://github.com/facebook/react-native/pull/7974

Differential Revision: D3401208

Pulled By: nicklockwood

fbshipit-source-id: 7c807339ba512e05f848036fb40840cf3fee8df6

Libraries/NavigationExperimental/Reducer/NavigationScenesReducer.js

@@ -122,7 +122,7 @@ function NavigationScenesReducer(
     };
     invariant(
       !nextKeys.has(key),
-      `navigationState.routes[${index}].key "${key}" conflicts with` +
+      `navigationState.routes[${index}].key "${key}" conflicts with ` +
         'another route!'
     );
     nextKeys.add(key);