[cloud][docs] Must use the count_tag option with exact_count (#23167)

Fixed an example in ec2 module. The example does not work without
count_tag option.

lib/ansible/modules/cloud/amazon/ec2.py

@@ -357,6 +357,8 @@ EXAMPLES = '''
         volume_size: 8
     vpc_subnet_id: subnet-29e63245
     assign_public_ip: yes
+    count_tag:
+      Name: dbserver
     exact_count: 1
 
 # Multiple groups example