Fixed typo in test file comment.

Author: carltongibson

tests/test_sync_contextvars.py

@@ -61,7 +61,7 @@ def test_async_to_sync_contextvars():
     present in the called context, and that any changes in the called context
     are then propagated back to the calling context.
     """
-    # Define sync function
+    # Define async function
     async def async_function():
         await asyncio.sleep(1)
         assert foo.get() == "bar"