AFAIK you can not create an A record at the zone apex, only an AWS-specific Alias type.
The Alias can refer to an ELB, S3 website, CloudFront distribution or another Route 53 record set. You have a couple of options:
a) put your instance behind an ELB, and create example.com as an Alias record pointing to your ELB. Or,
b) Create example.com as an Alias which points to the www.example.com CNAME.