Update creating-a-vpc.md

[bareeqakhan]

Removed 12th point from 'Public and Private subnets' section since 'AssignIpv6AddressOnCreation' is supported in public subnets in CFN.

Issue #, if available:
In the CFN template provided for VPC with public and private subnets (IPv4 and IPv6), e.g. https://s3.us-west-2.amazonaws.com/amazon-eks/cloudformation/2020-10-29/amazon-eks-ipv6-vpc-public-private-subnets.yaml a limitation has been mentioned -
"Note: AssignIpv6AddressOnCreation: true is required but a CFN limitation right now, need to add this manually"
Due to which we need to manually enable the auto-assign IPv6 address option for the public subnets that were created by the template (as per point 12)
Since this limitation has already been addressed by CFN, so 'AssignIpv6AddressOnCreation' property is supported in public subnets as well and hence manually performing this step is not required now

Description of changes:

• Please update the CFN template to include 'AssignIpv6AddressOnCreation: true' in public subnets as well {See attachment for updated template EKS-IPv4-IPv6.txt }
• Remove point 12 that describes how to manually enable the auto-assign IPv6 address option for the public subnets

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