Fixed describe() in core/detector.py to be async

src/ophyd_async/core/detector.py

@@ -240,7 +240,7 @@ async def read(self) -> Dict[str, Reading]:
         # All data is in StreamResources, not Events, so nothing to output here
         return {}
 
-    def describe(self) -> Dict[str, Descriptor]:
+    async def describe(self) -> Dict[str, Descriptor]:
         return self._describe
 
     @AsyncStatus.wrap