Fix typo

Author: jcs090218

README.md

@@ -220,7 +220,7 @@ Function takes 2 arguments, `node` and `offset`.
 
 * `node` - the targeting tree-sitter node; in this example, `block` will be the
 targeting node.
-* `offset` - (optiona) a cons consist of two integers. This is handy when you have
+* `offset` - (optional) a cons consist of two integers. This is handy when you have
 a similar rule with little of positioning adjustment.
 
 `tree-sitter-[lang]` parsers are generally integrated with different authors,