Fix the signature of multiprocessing.set_executable (GH-31276)

Automerge-Triggered-By: GH:merwok
(cherry picked from commit 4f9386661d)

Co-authored-by: Géry Ogam <gery.ogam@gmail.com>

Co-authored-by: Géry Ogam <gery.ogam@gmail.com>

Doc/library/multiprocessing.rst

@@ -1047,9 +1047,9 @@ Miscellaneous
 
    .. versionadded:: 3.4
 
-.. function:: set_executable()
+.. function:: set_executable(executable)
 
-   Sets the path of the Python interpreter to use when starting a child process.
+   Set the path of the Python interpreter to use when starting a child process.
    (By default :data:`sys.executable` is used).  Embedders will probably need to
    do some thing like ::