fix(templates): replace dash with underscore

[itsJohnySmith]

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Description

Replaced dash with underscore to prevent "not a valid identifier" error in bash.

Motivation and Context

• Once you deploy minio helm chart, you are provided with instructions on how to connect to minio instance running in pod.
• These instruction include exporting environmental variable MC_HOST_{{ template "minio.fullname" . }}-local.
• Dash in variable name causes bash to fail with error: not a valid identifier
• details in wrongly closed issue MC_HOST_minio-local is not a valid identifier #19331

How to test this PR?

Deploy minio helm chart and follow instructions printed by helm from NOTES.txt

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)
• Unit tests added/updated
• Internal documentation updated
• Create a documentation update request here