Switch HTTP links to use HTTPS

Author: amartini51

TSPL.docc/LanguageGuide/Macros.md

@@ -520,7 +520,7 @@ automatically includes a dependency on SwiftSyntax.
 If you're adding macros to an existing project,
 add a dependency on SwiftSyntax in your `Package.swift` file:
 
-[SwiftSyntax]: http://github.com/apple/swift-syntax/
+[SwiftSyntax]: https://github.com/apple/swift-syntax/
 
 ```swift
 dependencies: [

TSPL.docc/ReferenceManual/Declarations.md

@@ -3582,7 +3582,7 @@ Call the `externalMacro(module:type:)` macro from the Swift standard library,
 passing in the name of a type that contains the macro's implementation,
 and the name of the module that contains that type.
 
-[SwiftSyntax]: http://github.com/apple/swift-syntax/
+[SwiftSyntax]: https://github.com/apple/swift-syntax/
 
 Macros can be overloaded,
 following the same model used by functions.