Update Doc/library/asyncio-extending.rst

Co-authored-by: Hugo van Kemenade <1324225+hugovk@users.noreply.github.com>

Author: sobolevn

Doc/library/asyncio-extending.rst

@@ -17,7 +17,7 @@ classes. Asyncio has helpers that could be used to simplify this task.
 
 .. _writing_custom_event_loop:
 
-Writing a Custom Event Loop
+Writing a custom event loop
 ===========================
 
 :class:`asyncio.AbstractEventLoop` declares very many methods.