adding note about the case-sensitivity of filenames

[sdwheeler]

Description

Received a comment via LiveFyre for the Set-AzureRmVMCustomScriptExtension cmdlet reference. The comment stated "This attribute appears to be case sensitive especially when pulling the file from an Azure Blob storage container" for the -FileName attribute.

The parameter name is not case sensitive, but the value passed may be case sensitive based on the type of storage used. Updated the description of this parameter to document this.

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