Update S3 Event to working Go file (#212)

* Update S3 Event to working Go file

Example provided does not work without changing the script slightly...

* Update README_S3.md

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events/README_S3.md

@@ -4,17 +4,27 @@ The following is a sample class and Lambda function that receives Amazon S3 even
 
 ```go
 
+// main.go
+package main
+
 import (
-    "fmt"
-    "context"
-    "github.com/aws/aws-lambda-go/events"
+	"fmt"
+	"context"
+	"github.com/aws/aws-lambda-go/lambda"
+	"github.com/aws/aws-lambda-go/events"
 )
 
 func handler(ctx context.Context, s3Event events.S3Event) {
-    for _, record := range s3Event.Records {
-        s3 := record.S3
-        fmt.Printf("[%s - %s] Bucket = %s, Key = %s \n", record.EventSource, record.EventTime, s3.Bucket.Name, s3.Object.Key) 
-    }
+  	for _, record := range s3Event.Records {
+      		s3 := record.S3
+      		fmt.Printf("[%s - %s] Bucket = %s, Key = %s \n", record.EventSource, record.EventTime, s3.Bucket.Name, s3.Object.Key)
+  	}
+}
+
+
+func main() {
+	// Make the handler available for Remote Procedure Call by AWS Lambda
+	lambda.Start(handler)
 }
 
 ```