Move the example

The example under `CreateFromFile(String)` should be moved to `CreateFromFile(String, FileMode, String)`, because it actually uses that overload of the method

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* ID: 25bccff2-0202-40c0-278d-9f791e353b93
* Version Independent ID: a4d0765b-0488-c971-977c-cf4960b58b88
* Content: [MemoryMappedFile.CreateFromFile Method (System.IO.MemoryMappedFiles)](https://docs.microsoft.com/en-us/dotnet/api/system.io.memorymappedfiles.memorymappedfile.createfromfile?view=netframework-4.7.2)
* Content Source: [xml/System.IO.MemoryMappedFiles/MemoryMappedFile.xml](https://github.com/dotnet/dotnet-api-docs/blob/master/xml/System.IO.MemoryMappedFiles/MemoryMappedFile.xml)
* Product: **dotnet-api**
* GitHub Login: @dotnet-bot
* Microsoft Alias: **dotnetcontent**

打开 xml/System.IO.MemoryMappedFiles/MemoryMappedFile.xml，找到 CreateFromFile(String) 下的示例。将其移动到 CreateFromFile(String, FileMode, String) 下，因为该示例使用的是这个重载。完成标准是：示例出现在匹配的 API 重载下，并且不再出现在单参数方法下。

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