[APINotes] Match the SwiftName for NSXMLDTDKind to the headers. (#6901)

In the SDK shipped with Swift 3, NSXMLNodeKind's enumerator
NSXMLDTDKind was mistakenly annotated with `NS_SWIFT_NAME(DTDKind)`
instead of `NS_SWIFT_NAME(dtd)`. We'd like to fix that in a later
version of the SDK, but we need to maintain Swift 3 compatibility on
the off-chance someone is using the bad name. If we /do/ decide to
change this, we can undo this change (going back to an API note
SwiftName of 'dtd') and add a Swift-3-versioned SwiftName of 'DTDKind'
instead.

Author: jrose-apple

apinotes/Foundation.apinotes

@@ -1555,7 +1555,7 @@ Enumerators:
 - Name: NSXMLTextKind
   SwiftName: text
 - Name: NSXMLDTDKind
-  SwiftName: dtd
+  SwiftName: DTDKind
 - Name: NSXMLEntityDeclarationKind
   SwiftName: entityDeclaration
 - Name: NSXMLAttributeDeclarationKind