Merge pull request #162 from hackaugusto/hacka-typo-sample-code

typo: fixed keyword on sample code

book/src/formality_core/parse.md

@@ -13,7 +13,7 @@ For enums, the grammar is placed in `#[grammar]` attributes on each variant:
 
 ```rust
 #[term]
-struct MyEnum {
+enum MyEnum {
     #[grammar( /* grammar here */ )]
     Foo(...),
 }