Add token markup.
diff --git a/Doc/library/string.rst b/Doc/library/string.rst
index 3d1a3e8..4c3be4f 100644
--- a/Doc/library/string.rst
+++ b/Doc/library/string.rst
@@ -221,7 +221,7 @@
.. productionlist:: sf
replacement_field: "{" `field_name` ["!" `conversion`] [":" `format_spec`] "}"
- field_name: (`identifier` | `integer`) ("." `attribute_name` | "[" element_index "]")*
+ field_name: (`identifier` | `integer`) ("." `attribute_name` | "[" `element_index` "]")*
attribute_name: `identifier`
element_index: `integer`
conversion: "r" | "s"