fix exception message (#3944)

Author: loveyoupeng

src/main/java/rx/Observable.java

@@ -8523,7 +8523,7 @@ public final Subscription subscribe(Subscriber<? super T> subscriber) {
     static <T> Subscription subscribe(Subscriber<? super T> subscriber, Observable<T> observable) {
      // validate and proceed
         if (subscriber == null) {
-            throw new IllegalArgumentException("observer can not be null");
+            throw new IllegalArgumentException("subscriber can not be null");
         }
         if (observable.onSubscribe == null) {
             throw new IllegalStateException("onSubscribe function can not be null.");