docs: fix simple typo, schduling -> scheduling (#910)

There is a small typo in tests/test_observer.py.

Should read `scheduling` rather than `schduling`.

Signed-off-by: Tim Gates <tim.gates@iress.com>

tests/test_observer.py

@@ -137,7 +137,7 @@ def mocked_start():
     observer.start()
     assert len(observer.emitters) == 0
 
-    # Re-schduling the watch should work
+    # Re-scheduling the watch should work
     observer.schedule(None, '')
     assert len(observer.emitters) == 1