Review comment

Co-authored-by: Julian Ganz <[email protected]>

src/functional/optics.md

@@ -350,7 +350,7 @@ Thus we have the Serde API:
 1. Each type to be serialized implements `Deserialize` or `Serialize`,
    equivalent to the `Serde` class
 1. They get a type (well two, one for each direction) implementing the `Visitor`
-   trait, which are usually (but not always) is done through code generated by
+   trait, which is usually (but not always) done through code generated by
    a derive macro. This contains the logic to construct or destruct between the
    data type and the format of the Serde data model.
 1. The type implementing the `Deserializer` trait handles all details specific