Keep the ignore

Author: hugovk

Doc/tools/.nitignore

@@ -36,6 +36,7 @@ Doc/library/smtplib.rst
 Doc/library/socket.rst
 Doc/library/ssl.rst
 Doc/library/stdtypes.rst
+Doc/library/subprocess.rst
 Doc/library/termios.rst
 Doc/library/test.rst
 Doc/library/tkinter.rst