Update managed-identity-based-authentication.md

There exists no ReadAsAsync method on an HttpContent. Rather, this should use the extension method ReadFromJsonAsync in order to pass in a type and read the result.

Author: WhitWaldo

articles/cosmos-db/managed-identity-based-authentication.md

@@ -182,7 +182,7 @@ namespace Monitor
             var result = await httpClient.PostAsync(endpoint, new StringContent(""));
 
             // Get the result back as a DatabaseAccountListKeysResult.
-            DatabaseAccountListKeysResult keys = await result.Content.ReadAsAsync<DatabaseAccountListKeysResult>();
+            DatabaseAccountListKeysResult keys = await result.Content.ReadFromJsonAsync<DatabaseAccountListKeysResult>();
 
             log.LogInformation("Starting to create the client");