update demo target's name 😂

Author: sunlubo

Package.swift

@@ -19,8 +19,9 @@ let package = Package(
             name: "SwiftSDL2"
         ),
         .target(
-            name: "Demo",
-            dependencies: ["SwiftSDL2"]
+            name: "SwiftSDL2Demo",
+            dependencies: ["SwiftSDL2"],
+            path: "Sources/Demo"
         ),
         .testTarget(
             name: "SwiftSDL2Tests",