[Storage] Fix metadata typehint in file-share

[vincenttran-msft]

This change is to fix the following behavior:
:keyword dict(str,str) metadata: renders as:

:keyword dict[str, str] renders as:

(loses ability to imply optionality??)

So now we are moving towards:

:keyword metadata:
            Name-value pairs associated with the share as metadata.
:paramtype metadata: Optional[dict[str, str]]

To get the correct rendering:

[azure-sdk]

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azure-storage-file-share