fix doc linting

Author: Berthin Torres

Doc/library/netrc.rst

@@ -19,7 +19,7 @@ the Unix :program:`ftp` program and other FTP clients.
 
    A :class:`~netrc.netrc` instance or subclass instance encapsulates data from  a netrc
    file. The initialization argument, if present, specifies the file to parse. If no
-   argument is given, it will look for the file path in the `NETRC` environment variable.
+   argument is given, it will look for the file path in the :envvar:`NETRC` environment variable.
    If that is not set, it defaults to reading the file :file:`.netrc` in the user's home
    directory -- as determined by :func:`os.path.expanduser`. If the file cannot be found,
    a :exc:`FileNotFoundError` exception will be raised.