Update RFC0020-DefaultFileEncoding.md

* Removed Default, Legacy, OEM, and Unknown from the enum set. Default and OEM might need to be brought back (and I'm filing an issue to discuss this), Legacy was a typo (WindowsLegacy was already listed), and Unknown makes no sense as an input value. 
* Fixed another instance where Legacy should have been WindowsLegacy

Author: joeyaiello

1-Draft/RFC0020-DefaultFileEncoding.md

@@ -51,20 +51,16 @@ The following is the complete list of `FileSystemCmdletProviderEncoding` members
 * BigEndianUnicode
 * BigEndianUTF32
 * Byte
-* Default
-* Legacy
-* Oem
 * String
 * Unicode
-* Unknown
 * UTF32
 * UTF7
 * UTF8
 * UTF8NoBOM
 
 When `$PSDefaultEncoding` is set to `UTF8NoBOM`, the file shall be created with UTF8 encoding and no BOM shall be written.
 
-When `$PSDefaultEncoding` is set to `Legacy`, the behavior shall be:
+When `$PSDefaultEncoding` is set to `WindowsLegacy`, the behavior shall be:
 
 ```
 CmdletName          Encoding