Mistake in "Important" block

The "Important" block which discusses the fact that the StreamReader implements IDisposable suggests that one approach for handling this cleanly is a "try / catch" block. This should be "try / finally".

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* ID: 73a7a6b6-a052-c529-6ce3-86641aafac19
* Version Independent ID: 50f2a627-e593-1b0b-da0c-ac0fa08ac1db
* Content: [StreamReader Class (System.IO)](https://docs.microsoft.com/en-us/dotnet/api/system.io.streamreader?f1url=https%3A%2F%2Fmsdn.microsoft.com%2Fquery%2Fdev15.query%3FappId%3DDev15IDEF1%26l%3DEN-US%26k%3Dk(System.IO.StreamReader);k(TargetFrameworkMoniker-.NETFramework,Version%3Dv4.6.1);k(DevLang-csharp)%26rd%3Dtrue&view=netframework-4.7.2)
* Content Source: [xml/System.IO/StreamReader.xml](https://github.com/dotnet/dotnet-api-docs/blob/master/xml/System.IO/StreamReader.xml)
* Product: **dotnet-api**
* GitHub Login: @mairaw
* Microsoft Alias: **mairaw**