Docs: Add cross-reference for positional_item in the calls productionlist (pythonGH-129977)

Add missing hyperlink for `positional_item`

Author: HarryLHW

Doc/reference/expressions.rst

@@ -1023,7 +1023,7 @@ series of :term:`arguments <argument>`:
                 :   ["," `keywords_arguments`]
                 : | `starred_and_keywords` ["," `keywords_arguments`]
                 : | `keywords_arguments`
-   positional_arguments: positional_item ("," positional_item)*
+   positional_arguments: `positional_item` ("," `positional_item`)*
    positional_item: `assignment_expression` | "*" `expression`
    starred_and_keywords: ("*" `expression` | `keyword_item`)
                 : ("," "*" `expression` | "," `keyword_item`)*