RSACryptoServiceProvider in sample code instead of RSA

The documentation says Key property contains instance of RSA or DSA object. However, the sample code still uses RSACryptoServiceProvider type cast as was used in "old" .NET. This is very confusing for the users.
```
EncryptFile(originalFile, (RSACryptoServiceProvider)cert.PublicKey.Key);
```

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#### Document Details

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* ID: 16afc3e9-947b-e39d-fe6a-12be630c986f
* Version Independent ID: 2d6b18ba-53ee-0f08-e981-bfaa66854293
* Content: [PublicKey Class (System.Security.Cryptography.X509Certificates)](https://docs.microsoft.com/en-us/dotnet/api/system.security.cryptography.x509certificates.publickey?view=netframework-4.7.2)
* Content Source: [xml/System.Security.Cryptography.X509Certificates/PublicKey.xml](https://github.com/dotnet/dotnet-api-docs/blob/master/xml/System.Security.Cryptography.X509Certificates/PublicKey.xml)
* Product: **dotnet-api**
* GitHub Login: @mairaw
* Microsoft Alias: **mairaw**