fix: add missing word (#3237)

Author: Elliot67

docs/api/authentication/strategy.md

@@ -10,7 +10,7 @@ An authentication strategy is any object or class that implements at least an [a
 - [LocalStrategy](./local.md) in `@feathersjs/authentication-local`
 - [OAuthStrategy](./oauth.md) in `@feathersjs/authentication-oauth`
 
-More details on how to customize existing strategies can be found in their API documentation. This section describes the common methods for all authentication strategies and how a custom authentication strategy can implemented.
+More details on how to customize existing strategies can be found in their API documentation. This section describes the common methods for all authentication strategies and how a custom authentication strategy can be implemented.
 
 ## setName(name)