Update test instructions with proper cp command
Previously renames libgdnative_test.so to lib, instead of putting
it in the folder lib.
@@ -31,7 +31,7 @@ Have a look at the [issues](https://github.com/GodotNativeTools/godot-rust/issue
```
cd test
cargo build
-cp ../target/debug/libgdnative_test.so ./project/lib
+cp ../target/debug/libgdnative_test.so ./project/lib/
godot --path ./project
```
