fix confusing typo
This document describes the `Vocab` class, not the `Span` class.
@@ -124,7 +124,7 @@ p
+cell #[code Lexeme]
+cell The lexeme indicated by the given ID.
-+h(2, "iter") Span.__iter__
++h(2, "iter") Vocab.__iter__
+tag method
p Iterate over the lexemes in the vocabulary.
