Add missing slash.

Summary:
This should be there for consistency.
Closes #11357

Differential Revision: D4294530

Pulled By: kentaromiura

fbshipit-source-id: 6219a7c69a65dde21d96bc032fc9a327386a85ca

Author: cpojer

jest-preset.json

@@ -8,7 +8,7 @@
   },
   "moduleNameMapper": {
     "^[./a-zA-Z0-9$_-]+\\.(bmp|gif|jpg|jpeg|png|psd|svg|webp)$": "RelativeImageStub",
-    "^React$": "<rootDir>node_modules/react"
+    "^React$": "<rootDir>/node_modules/react"
   },
   "modulePathIgnorePatterns": [
     "<rootDir>/node_modules/react-native/Libraries/react-native/",