correctly define what 'fill' could be; thanks to Leland Hulbert from docs@

Doc/library/string.rst

@@ -292,7 +292,7 @@ The general form of a *standard format specifier* is:
 
 .. productionlist:: sf
    format_spec: [[`fill`]`align`][`sign`][#][0][`width`][,][.`precision`][`type`]
-   fill: <a character other than '}'>
+   fill: <a character other than '{' or '}'>
    align: "<" | ">" | "=" | "^"
    sign: "+" | "-" | " "
    width: `integer`