update KMS README.md to set approle env

[poornas]

Description

README is missing settings for MINIO_SSE_VAULT_AUTH_TYPE

Motivation and Context

for usability

Regression

No

How Has This Been Tested?

• follow kms settings in readme and minio server should start up with vault enabled.
  aws s3api put-object --bucket testb1 --key s3copy/a/b/c --body /tmp/smallfile --endpoint-url http://localhost:9000 --server-side-encryption AES256
  should create the object

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added unit tests to cover my changes.
• I have added/updated functional tests in mint. (If yes, add mint PR # here: )
• All new and existing tests passed.

[vadmeste]

LGTM

[minio-ops]

Mint Automation

Test	Result
mint-tls.sh	✔️
mint-compression-xl.sh	✔️
mint-xl.sh	✔️
mint-large-bucket.sh	✔️
mint-compression-fs.sh	✔️
mint-worm.sh	✔️
mint-fs.sh	✔️
mint-dist-xl.sh	✔️

[kannappanr]

LGTM

[codecov]

Codecov Report

Merging #6978 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #6978      +/-   ##
==========================================
+ Coverage   52.22%   52.23%   +0.01%     
==========================================
  Files         271      271              
  Lines       42752    42752              
==========================================
+ Hits        22326    22331       +5     
+ Misses      18440    18437       -3     
+ Partials     1986     1984       -2

Impacted Files	Coverage Δ
cmd/posix.go	68.88% <0%> (+0.35%)	⬆️
cmd/fs-v1-helpers.go	68.8% <0%> (+0.61%)	⬆️

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