example code does not work with sample config file

the api used, `Environment.ExpandEnvironmentVariables(rawXmlString)` will only work when the config file has the following `<add key="COMPUTERNAME" value="%COMPUTERNAME%" />`

Environment.ExpandEnvironmentVariables only expands % delimited tokens.

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#### Document Details

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* ID: 9fdfd344-8a4b-f23d-c040-ee6982322041
* Version Independent ID: d9b615a6-1846-e521-b671-c7d384be68cb
* Content: [ConfigurationBuilder Class (System.Configuration)](https://docs.microsoft.com/en-us/dotnet/api/system.configuration.configurationbuilder?view=netframework-4.7.1&WT.mc_id=azure-blog-antchu)
* Content Source: [xml/System.Configuration/ConfigurationBuilder.xml](https://github.com/dotnet/dotnet-api-docs/blob/master/xml/System.Configuration/ConfigurationBuilder.xml)
* Product: **dotnet-api**
* GitHub Login: @dotnet-bot
* Microsoft Alias: **dotnetcontent**