Update SignedAPKAndroid.md

Summary:
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Add another keystore's settings path.

`+ or android/gradle.properties`
Closes https://github.com/facebook/react-native/pull/15846

Differential Revision: D5785326

Pulled By: hramos

fbshipit-source-id: 5bc2acbd53c7ce7fe4e866b8855c3ca8cd4d60dd

docs/SignedAPKAndroid.md

@@ -26,7 +26,7 @@ _Note: Remember to keep your keystore file private and never commit it to versio
 ### Setting up gradle variables
 
 1. Place the `my-release-key.keystore` file under the `android/app` directory in your project folder.
-2. Edit the file `~/.gradle/gradle.properties` and add the following (replace `*****` with the correct keystore password, alias and key password),
+2. Edit the file `~/.gradle/gradle.properties` or `android/gradle.properties` and add the following (replace `*****` with the correct keystore password, alias and key password),
 
 ```
 MYAPP_RELEASE_STORE_FILE=my-release-key.keystore