Fix: Update Javadoc reference in ExternalAccountCredentials (#1763)

The Javadoc for ExternalAccountCredentials.java incorrectly referred to
`GoogleCredential.Builder`. This commit corrects the reference to
`GoogleCredentials.Builder`.

Author: lsirac

oauth2_http/java/com/google/auth/oauth2/ExternalAccountCredentials.java

@@ -763,7 +763,7 @@ public abstract static class Builder extends GoogleCredentials.Builder {
     @Nullable protected ServiceAccountImpersonationOptions serviceAccountImpersonationOptions;
 
     /* The field is not being used and value not set. Superseded by the same field in the
-    {@link GoogleCredential.Builder}.
+    {@link GoogleCredentials.Builder}.
     */
     @Nullable @Deprecated protected String universeDomain;