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>
1 file changed