gh-116881: Remove erroneous or redundant grammar NULL (GH-116885)

In Lexical Analysis f-strings section, NULL in the description
of 'literal character' means '\0'.  In the format_spec grammar
production, it is wrong with that meaning and redundant if
instead interpreted as <nothing>.  Remove it there.

Doc/reference/lexical_analysis.rst

@@ -733,7 +733,7 @@ for the contents of the string is:
                :   ("," `conditional_expression` | "," "*" `or_expr`)* [","]
                : | `yield_expression`
    conversion: "s" | "r" | "a"
-   format_spec: (`literal_char` | NULL | `replacement_field`)*
+   format_spec: (`literal_char` | `replacement_field`)*
    literal_char: <any code point except "{", "}" or NULL>
 
 The parts of the string outside curly braces are treated literally,