Fix grammar error in RxJavaHooks javadocs (#4191)

Author: bryant1410

src/main/java/rx/plugins/RxJavaHooks.java

@@ -275,7 +275,7 @@ public static void clear() {
     }
 
     /**
-     * Prevents changing a hooks.
+     * Prevents changing the hooks.
      */
     public static void lockdown() {
         lockdown = true;