fix: Use .git protocol extension for docc plugin url

Author: maximkrouk

Package.swift

@@ -12,7 +12,7 @@ let package = Package(
   ],
   dependencies: [
     .package(
-      url: "https://github.com/apple/swift-docc-plugin",
+      url: "https://github.com/apple/swift-docc-plugin.git",
       from: "1.3.0"
     ),
   ],