Amazon SNS service -> service name includes topic

Hi,

While looking at our dependencies, I found that Amazon SNS service's name is equal to `subtype/topic_id`. You can imagine having multiple topic what kind of graph is rendered. I think having 1 service SNS would be enough for most cases.

The resource is set [here](https://github.com/elastic/apm-agent-ruby/blob/5f6fc2bad4857f43bbb0b177a97c6149cbd78650/lib/elastic_apm/spies/sns.rb#L66-L71) and I would gladly open a PR to change it.

Thanks.