Use list_objects_v2 in README

[nyoungstudios]

Description of Change

• Updates README documentation to use list_objects_v2 instead of list_objects since that is what the AWS documentation recommends (and list_objects_v2 supports automatic pagination with the client.get_paginator function used in the sample). Should I add a comment or anything in the readme?
• list_objects documentation recommends to use list_objects_v2 documentation

Assumptions

• none

Checklist for All Submissions

• I have added change info to CHANGES.rst
  • Do you want me to add something to the change list for a documentation change?
• If this is resolving an issue (needed so future developers can determine if change is still necessary and under what conditions) (can be provided via link to issue with these details):
  • Detailed description of issue
  • Alternative methods considered (if any)
  • How issue is being resolved
  • How issue can be reproduced
• If this is providing a new feature (can be provided via link to issue with these details):
  • Detailed description of new feature
  • Why needed
  • Alternatives methods considered (if any)

Checklist when updating botocore and/or aiohttp versions

• I have read and followed CONTRIBUTING.rst
• I have updated test_patches.py where/if appropriate (also check if no changes necessary)
• I have ensured that the awscli/boto3 versions match the updated botocore version