Add comma to prevent build error when react.gradle is uncommented

Summary:
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At the moment, if you uncomment the react specific code in `build.gradle` you'll get a compilation error. Adding a comma prevents this error.

No tests are necessary. This is a simple fix.

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Small pull requests are much easier to review and more likely to get merged. Make sure the PR does only one thing, otherwise please split it.

Make sure all **tests pass** on both [Travis][3] and [Circle CI][4]. PRs that break tests are unlikely to be merged.

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[1]: https://medium.com/martink
Closes https://github.com/facebook/react-native/pull/13212

Differential Revision: D4802065

Pulled By: javache

fbshipit-source-id: 0ddfe16241381d7e4ac0e48be1bada21d9df2068

local-cli/templates/HelloWorld/android/app/build.gradle

@@ -58,7 +58,7 @@ import com.android.build.OutputFile
  *   inputExcludes: ["android/**", "ios/**"],
  *
  *   // override which node gets called and with what additional arguments
- *   nodeExecutableAndArgs: ["node"]
+ *   nodeExecutableAndArgs: ["node"],
  *
  *   // supply additional arguments to the packager
  *   extraPackagerArgs: []