Update README: fix godot-cpp issue tracker url

(cherry picked from commit 8a535d0)

README.md

@@ -58,7 +58,7 @@ first-party `godot-cpp` extension.
 Some compatibility breakage is to be expected as GDExtension and `godot-cpp`
 get more used, documented, and critical issues get resolved. See the
 [Godot issue tracker](https://github.com/godotengine/godot/issues?q=is%3Aissue+is%3Aopen+label%3Atopic%3Agdextension)
-and the [godot-cpp issue tracker](https://github.com/godotengine/godot/issues)
+and the [godot-cpp issue tracker](https://github.com/godotengine/godot-cpp/issues)
 for a list of known issues, and be sure to provide feedback on issues and PRs
 which affect your use of this extension.