Typo Fix (#20087)

Changed 'mmutability' to 'immutability'

Author: thwpike

src/Storage/Storage.Management/help/Set-AzStorageAccount.md

@@ -422,7 +422,7 @@ Enabled
 
 This command updates a Storage account by set PublicNetworkAccess as enabled.
 
-### Example 19: Update account level  mmutability policy
+### Example 19: Update account level immutability policy
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