Fix typo
This makes the sentence more cohesive.
@@ -42,7 +42,7 @@ Filename: <a class=file-link href="https://github.com/gtk-rs/gtk4-rs/blob/master
> Describing objects with two structs is a peculiarity coming from how GObjects are defined in C.
> `imp::CustomButton` handles the state of the GObject and the overridden virtual methods.
-> `CustomButton` determines the exposed methods be determining the implemented traits and adding methods itself.
+> `CustomButton` determines the exposed methods from the implemented traits and added methods.
## Adding Functionality
