Fix error msg to comply with a Go linter

[jjngx]

Proposed changes

This PR makes error massages in the secret pkg comply with the linter:

Before:

➜  kubernetes-ingress git:(main) staticcheck ./...
...
internal/k8s/secrets/validation.go:81:10: error strings should not be capitalized (ST1005)
internal/k8s/secrets/validation.go:84:10: error strings should not be capitalized (ST1005)
internal/k8s/secrets/validation.go:115:10: error strings should not be capitalized (ST1005)
internal/k8s/secrets/validation.go:119:10: error strings should not be capitalized (ST1005)
internal/k8s/secrets/validation.go:152:9: error strings should not be capitalized (ST1005)

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
• I have rebased my branch onto main
• I will ensure my PR is targeting the main branch and pulling from my branch from my own fork