Fix the span doc typo
Fix the typo in the span API doc.
It explains the `end` of the span as the `start_char` description.
@@ -25,7 +25,7 @@ p A slice from a #[code Doc] object.
+row
+cell #[code start_char]
+cell int
- +cell The character offset for the end of the span.
+ +cell The character offset for the start of the span.
+row
+cell #[code end_char]
