doc: Remove backslashes in doctest grammar docs (GH-29346)

(cherry picked from commit 22860db)

Co-authored-by: George Zhang <[email protected]>

Author: miss-islington

Doc/library/doctest.rst

@@ -697,10 +697,10 @@ special Python comments following an example's source code:
 
 .. productionlist:: doctest
    directive: "#" "doctest:" `directive_options`
-   directive_options: `directive_option` ("," `directive_option`)\*
+   directive_options: `directive_option` ("," `directive_option`)*
    directive_option: `on_or_off` `directive_option_name`
-   on_or_off: "+" \| "-"
-   directive_option_name: "DONT_ACCEPT_BLANKLINE" \| "NORMALIZE_WHITESPACE" \| ...
+   on_or_off: "+" | "-"
+   directive_option_name: "DONT_ACCEPT_BLANKLINE" | "NORMALIZE_WHITESPACE" | ...
 
 Whitespace is not allowed between the ``+`` or ``-`` and the directive option
 name.  The directive option name can be any of the option flag names explained