Doc: change 'Posix' for 'POSIX' (GH-20001)

(cherry picked from commit 65460565df99fbda6a74b6bb4bf99affaaf8bd95)

Co-authored-by: Mathieu Dupuy <mathieu.dupuy@doctolib.com>
diff --git a/Doc/library/subprocess.rst b/Doc/library/subprocess.rst
index cce7da1..0b692b4 100644
--- a/Doc/library/subprocess.rst
+++ b/Doc/library/subprocess.rst
@@ -755,14 +755,14 @@
 
 .. method:: Popen.terminate()
 
-   Stop the child. On Posix OSs the method sends SIGTERM to the
+   Stop the child. On POSIX OSs the method sends SIGTERM to the
    child. On Windows the Win32 API function :c:func:`TerminateProcess` is called
    to stop the child.
 
 
 .. method:: Popen.kill()
 
-   Kills the child. On Posix OSs the function sends SIGKILL to the child.
+   Kills the child. On POSIX OSs the function sends SIGKILL to the child.
    On Windows :meth:`kill` is an alias for :meth:`terminate`.