fix on-intersect attribute generator

Author: gazpachoking

src/datastar_py/attributes.py

@@ -642,6 +642,8 @@ def focus(self) -> Self:
 
 
 class OnIntersectAttr(BaseAttr, TimingMod, DelayMod, ViewtransitionMod):
+    _attr = "on-intersect"
+
     @property
     def once(self) -> Self:
         """Only trigger the event listener once."""