Fix typo in documentation of AbstractEventLoopPolicy.set_child_watcher() (GH-11369)


`set_child_watcher()` *sets* the watcher.
(cherry picked from commit 1b29c03c95dbffa05f2bac0f8f1a36b21606a504)

Co-authored-by: sth <sth.dev@tejp.de>
diff --git a/Doc/library/asyncio-policy.rst b/Doc/library/asyncio-policy.rst
index cab2593..07842da 100644
--- a/Doc/library/asyncio-policy.rst
+++ b/Doc/library/asyncio-policy.rst
@@ -81,7 +81,7 @@
 
    .. method:: set_child_watcher(watcher)
 
-      Get the current child process watcher to *watcher*.
+      Set the current child process watcher to *watcher*.
 
       This function is Unix specific.