Add 'a' and 'A' format specifiers

doc/syntax.rst

@@ -80,7 +80,7 @@ The general form of a *standard format specifier* is:
    sign: "+" | "-" | " "
    width: `integer` | "{" `arg_id` "}"
    precision: `integer` | "{" `arg_id` "}"
-   type: `int_type` | "c" | "e" | "E" | "f" | "F" | "g" | "G" | "p" | "s"
+   type: `int_type` | "a" | "A" | "c" | "e" | "E" | "f" | "F" | "g" | "G" | "p" | "s"
    int_type: "b" | "B" | "d" | "n" | "o" | "x" | "X"
 
 The *fill* character can be any character other than '{' or '}'.  The presence