Fix a typo in subprocess documentation (GH-25426)

Fix a typo per conversation with vstinner on IRC: in posix => on posix.

Doc/library/subprocess.rst

@@ -543,7 +543,7 @@ functions.
 
    If *cwd* is not ``None``, the function changes the working directory to
    *cwd* before executing the child.  *cwd* can be a string, bytes or
-   :term:`path-like <path-like object>` object.  In POSIX, the function
+   :term:`path-like <path-like object>` object.  On POSIX, the function
    looks for *executable* (or for the first item in *args*) relative to *cwd*
    if the executable path is a relative path.