correctly define what 'fill' could be; thanks to Leland Hulbert from docs@
diff --git a/Doc/library/string.rst b/Doc/library/string.rst
index 1f8e230..fc0bd24 100644
--- a/Doc/library/string.rst
+++ b/Doc/library/string.rst
@@ -292,7 +292,7 @@
 
 .. 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`