Fix missing word.

Doc/library/os.rst

@@ -1837,7 +1837,7 @@ written in Python, such as a mail server's external command delivery program.
 
    Execute the command (a string) in a subshell.  This is implemented by calling
    the Standard C function :cfunc:`system`, and has the same limitations.
-   Changes :data:`sys.stdin`, etc. are not reflected in the environment of the
+   Changes to :data:`sys.stdin`, etc. are not reflected in the environment of the
    executed command.
 
    On Unix, the return value is the exit status of the process encoded in the