fix variable name typo in the help doc

src/scripts/prelink.js

@@ -67,7 +67,7 @@ try {
     console.log(
       '\033[95mreact-native-fetch-blob \033[97mwill not automatically add Android permissions after \033[92m0.9.4 '+
       '\033[97mplease run the following command if you want to add default permissions :\n\n' +
-      '\033[96m\tRNFB_ANDROID_PERMISSION=true react-native link \n')
+      '\033[96m\tRNFB_ANDROID_PERMISSIONS=true react-native link \n')
   }
 
   function checkVersion() {