Fix post-to-connection example for API Gateway (#9546)

Author: daniil-millwood

awscli/examples/apigatewaymanagementapi/post-to-connection.rst

@@ -4,7 +4,7 @@ The following ``post-to-connection`` example sends a message to a client that's
 
     aws apigatewaymanagementapi post-to-connection \
         --connection-id L0SM9cOFvHcCIhw= \
-        --data "Hello from API Gateway!" \
+        --data 'SGVsbG8gZnJvbSBBUEkgR2F0ZXdheSE=' \
         --endpoint-url https://aabbccddee.execute-api.us-west-2.amazonaws.com/prod
 
 This command produces no output.