fix ServiceAccount creation in helm chart

[kanagarajkm]

Description

Fixed the variable name on the templated and added name.

Motivation and Context

helm install is failing with the following error

helm.go:81: [debug] template: minio/templates/serviceaccount.yaml:5:11: executing "minio/templates/serviceaccount.yaml" at <include "minio.serviceAccountName" .>: error calling include: template: no template "minio.serviceAccountName" associated with template "gotpl"

How to test this PR?

helm install --namespace minio --set rootUser=rootuser,rootPassword=rootpass123 --generate-name ./helm/minio/ 

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Optimization (provides speedup with no functional changes)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• Fixes a regression (If yes, please add commit-id or PR # here)
• Documentation updated
• Unit tests added/updated

[minio-trusted]

Mint Automation

Test	Result
mint-large-bucket.sh	✔️
mint-fs.sh	✔️
mint-gateway-s3.sh	✔️
mint-erasure.sh	✔️
mint-dist-erasure.sh	✔️
mint-zoned.sh	✔️
mint-gateway-nas.sh	✔️
mint-compress-encrypt-dist-erasure.sh	✔️
Deleting image on docker hub
Deleting image locally