Backport from 3.2: fix formatting of syntax description of function definition.

Doc/reference/compound_stmts.rst

@@ -431,7 +431,7 @@ A function definition defines a user-defined function object (see section
    funcdef: "def" `funcname` "(" [`parameter_list`] ")" ":" `suite`
    dotted_name: `identifier` ("." `identifier`)*
    parameter_list: (`defparameter` ",")*
-                 : (  "*" `identifier` [, "**" `identifier`]
+                 : (  "*" `identifier` ["," "**" `identifier`]
                  : | "**" `identifier`
                  : | `defparameter` [","] )
    defparameter: `parameter` ["=" `expression`]