Tutorial: Fix indentation error in ThreadSafe flag example.

Author: peterhinch

v3/docs/TUTORIAL.md

@@ -1108,14 +1108,14 @@ async def foo(tsf):  # Periodically set the ThreadSafeFlag
         await asyncio.sleep(1)
         tsf.set()
 
-    def ready(tsf, poller):
-        r = (tsf, POLLIN)
-        poller.register(*r)
+def ready(tsf, poller):
+    r = (tsf, POLLIN)
+    poller.register(*r)
 
-        def is_rdy():
-            return r in poller.ipoll(0)
+    def is_rdy():
+        return r in poller.ipoll(0)
 
-        return is_rdy
+    return is_rdy
 
 async def test():
     tsf = asyncio.ThreadSafeFlag()