Fix variable assignment syntax in sample az cli commands for azure storage immutability policy

[AbhishekKrishna123]

Removed leading $ to fix the incorrect $etag=$(something) bash variable assignment syntax

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[v-dirichards]

@normesta

Can you review the proposed changes?

Important: When the changes are ready for publication, adding a #sign-off comment is the best way to signal that the PR is ready for the review team to merge.

#label:"aq-pr-triaged"
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[AbhishekKrishna123]

@normesta Looks like the document was written with AZ CLI in mind running on Powershell. I just assumed it was intended for bash. This command work on pwsh, and isn't wrong. I can close this PR if all commands we have in the docs for az cli are intended to be run on Powershell and not bash.

I see there is dedicated document explaining the differences in the environment.
https://learn.microsoft.com/en-us/cli/azure/get-started-tutorial-2-environment-syntax?view=azure-cli-latest&tabs=Bash