Docs show SNS as keyword instead of Topic for subscribing to SNS topics

The docs show the wrong keyword for subscribing to SNS topics.

What is shown:
```csharp
- Function: AccountReportGenerator
  Memory: 128
  Timeout: 30
  Sources:
    - SNS: AccountReportGeneratorTopic
```

What it should be:
```csharp
- Function: AccountReportGenerator
  Memory: 128
  Timeout: 30
  Sources:
    - Topic: AccountReportGeneratorTopic
```
