fix formatting in README
fix formatting in README

gerram/aws-sdk-ios-samples,JacobTorres/aws-sdk-ios-samples,JacobTorres/aws-sdk-ios-samples,awslabs/aws-sdk-ios-samples,mavieth/aws-sdk-ios-samples,borisyurkevich/aws-sdk-ios-samples,mavieth/aws-sdk-ios-samples,awslabs/aws-sdk-ios-samples,gerram/aws-sdk-ios-samples,borisyurkevich/aws-sdk-ios-samples