bpo-41959: Fix grammar around class asyncio.MultiLoopChildWatcher text (GH-22580) (#22865)

While translating the following document to Spanish we found there is a grammar issue on the original documentation.
(cherry picked from commit caff293)

Co-authored-by: Raúl Cumplido <[email protected]>

Co-authored-by: Raúl Cumplido <[email protected]>

Author: miss-islington

Doc/library/asyncio-policy.rst

@@ -219,7 +219,7 @@ implementation used by the asyncio event loop:
 
    This implementation registers a :py:data:`SIGCHLD` signal handler on
    instantiation. That can break third-party code that installs a custom handler for
-   `SIGCHLD`.  signal).
+   :py:data:`SIGCHLD` signal.
 
    The watcher avoids disrupting other code spawning processes
    by polling every process explicitly on a :py:data:`SIGCHLD` signal.