update string formatting grammar
diff --git a/Doc/library/string.rst b/Doc/library/string.rst
index 6ab555a..63caa4c 100644
--- a/Doc/library/string.rst
+++ b/Doc/library/string.rst
@@ -199,7 +199,7 @@
field_name: (`identifier` | `integer`) ("." `attribute_name` | "[" element_index "]")*
attribute_name: `identifier`
element_index: `integer`
- conversion: "r" | "s"
+ conversion: "r" | "s" | "a"
format_spec: <described in the next section>
In less formal terms, the replacement field starts with a *field_name*, which