Spelling fix in the generics overview doc (#2016)

Minor usage fix in generics doc

docs/design/generics/overview.md

@@ -223,8 +223,8 @@ external impl Song as Comparable {
 ```
 
 Implementations may be defined within the class definition itself or
-out-of-line. Implementations may optionally be start with the `external` keyword
-to say the members of the interface are not members of the class. Out-of-line
+out-of-line. Implementations may optionally start with the `external` keyword to
+say the members of the interface are not members of the class. Out-of-line
 implementations must be external. External implementations may be defined in the
 library defining either the class or the interface.