Fixed typing issue

Typing issue fixed as mentioned in #146 (comment)
and #121

Author: 28harishkumar

index.d.ts

@@ -17,9 +17,9 @@ interface IOptions {
   ios: {
     appName: string,
     imageName?: string,
-    supportsVideo: false,
-    maximumCallGroups: '1',
-    maximumCallsPerCallGroup: '1'
+    supportsVideo?: boolean,
+    maximumCallGroups?: string,
+    maximumCallsPerCallGroup?: string,
     ringtoneSound?: string,
   },
   android: {