bpo-44895: Update doc

Author: corona10

Doc/using/cmdline.rst

@@ -978,7 +978,7 @@ Debug-mode variables
 
 .. envvar:: PYTHONDUMPFILE
 
-   If set, Python will create a file at `$PYTHONDUMPFILE` as dump file
+   If set, Python will create a file named `$PYTHONDUMPFILE` as the dump file
    which is generated by :envvar:`PYTHONDUMPREFS`.
 
    Need Python configured with the :option:`--with-trace-refs` build option.