fixed typos (#589)

src/ophyd_async/core/_detector.py

@@ -194,7 +194,7 @@ def __init__(
         self._fly_status: WatchableAsyncStatus | None = None
         self._fly_start: float
         self._iterations_completed: int = 0
-        self._intial_frame: int
+        self._initial_frame: int
         self._last_frame: int
         super().__init__(name)
 
@@ -208,7 +208,7 @@ def writer(self) -> DetectorWriter:
 
     @AsyncStatus.wrap
     async def stage(self) -> None:
-        # Disarm the detector, stop filewriting.
+        # Disarm the detector, stop file writing.
         await self._check_config_sigs()
         await asyncio.gather(self.writer.close(), self.controller.disarm())
         self._trigger_info = None