Fix iOS setup step 4 (react-native-audio-toolkit#207)

Fix an issue where iOS usage description key was incorrect & not visible

Author: vaibhavhrt

docs/SETUP.md

@@ -111,8 +111,9 @@ React Native 0.59 and earlier:
     `libReactNativeAudioToolkit.a` from under Workspace.
     
 4. Add a usage description to **Info.plist**.
-    ```<key>Privacy - Microphone Usage Description</key>
-       <string>This app requires access to your microphone</string>
+    ```xml
+    <key>NSMicrophoneUsageDescription</key>
+    <string>This app requires access to your microphone</string>
     ```
 
 React Native 0.60 and later