TUTORIAL.md Clarify ThreadSafeFlag example.

Author: peterhinch

v3/docs/TUTORIAL.md

@@ -922,18 +922,33 @@ tim = Timer(1, freq=1, callback=cb)
 
 asyncio.run(foo())
 ```
-Another example ([posted by Damien](https://github.com/micropython/micropython/pull/6886#issuecomment-779863757)):
+An example [based on one posted by Damien](https://github.com/micropython/micropython/pull/6886#issuecomment-779863757)  
+Link pins X1 and X2 to test.
 ```python
+from machine import Pin, Timer
+import uasyncio as asyncio
+
 class AsyncPin:
     def __init__(self, pin, trigger):
         self.pin = pin
-        self.flag = ThreadSafeFlag()
+        self.flag = asyncio.ThreadSafeFlag()
         self.pin.irq(lambda pin: self.flag.set(), trigger, hard=True)
 
-    def wait_edge(self):
-        return self.flag.wait()
+    async def wait_edge(self):
+        await self.flag.wait()
+
+async def foo():
+    pin_in = Pin('X1', Pin.IN)
+    async_pin = AsyncPin(pin_in, Pin.IRQ_RISING)
+    pin_out = Pin('X2', Pin.OUT)  # Toggle pin to test
+    t = Timer(-1, period=500, callback=lambda _: pin_out(not pin_out()))
+    await asyncio.sleep(0)
+    while True:
+        await async_pin.wait_edge()
+        print('Got edge.')
+
+asyncio.run(foo())
 ```
-You then call `await async_pin.wait_edge()`.
 
 The current implementation provides no performance benefits against polling the
 hardware: other pending tasks may be granted execution first in round-robin