Fix some syntax errors

README.md

@@ -164,9 +164,9 @@ led = Pin(5, Pin.OUT)
 
 def callback(timer):
     global data, index
-    data[index] = utime.ticks_ms
+    data[index] = utime.ticks_ms()
     index = (index + 1) % 32
-    led(!led())
+    led(not led())
 
 # Run the callback once, after 100 ms
 timer.init(mode = Timer.ONE_SHOT, period = 100, callback = callback)