Update ec2_instance.py (ansible-collections#652)

Update ec2_instance.py

SUMMARY
vpc_subnet_id is not configurable under the network key, just tried a copy of the example and got this error->
TASK [manage_ec2_instances : Create EC2 instances for ansible node (control node)] **********************************************************************************************************
fatal: [localhost]: FAILED! => changed=false
  msg: No default subnet could be found - you must include a VPC subnet ID (vpc_subnet_id parameter) to create an instance

changing to match other examples
ISSUE TYPE

Docs Pull Request

COMPONENT NAME
ec2_instance
ADDITIONAL INFORMATION
n/a, only see one bad example here

Reviewed-by: Alina Buzachis <None>

plugins/modules/ec2_instance.py

@@ -451,10 +451,10 @@
     image_id: ami-123456
     exact_count: 5
     region: us-east-2
+    vpc_subnet_id: subnet-0123456
     network:
       assign_public_ip: yes
       security_group: default
-      vpc_subnet_id: subnet-0123456
     tags:
       foo: bar