Fix Route53 VPCs param name

Unable to get VPN information of private zone:
```ruby
route53 = AWS::Route53.new
hz = route53.hosted_zones['...']
p hz.vpcs #=> nil
```

vp_cs -> vpcs:
```ruby
route53 = AWS::Route53.new
hz = route53.hosted_zones['...']
p hz.vpcs
#=> [{:vpc_region=>"ap-northeast-1", :vpc_id=>"vpc-XXX"}]
```