Fix docs link (#3064)

Co-authored-by: Debora N. Ito <476307+debora-ito@users.noreply.github.com>
Co-authored-by: Zoe Wang <33073555+zoewangg@users.noreply.github.com>

Author: 3 people

docs/design/README.md

@@ -58,7 +58,7 @@ None
   client configuration objects. 
 * [Optional Usage](UseOfOptional.md) - Conventions governing the use of
   [java.util.Optional](https://docs.oracle.com/javase/8/docs/api/java/util/Optional.html).
-* [Completable Future Usage](UseOfCompletableFuture.java) - Conventions
+* [Completable Future Usage](UseOfCompletableFuture.md) - Conventions
   governing the use of
   [java.util.concurrent.CompletableFuture](https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/CompletableFuture.html).