Fix const string

Fix const string
NSString *const MJRefreshHeaderPullToRefresh = @"下拉可以刷新";

MJRefreshExample/MJRefreshExample/MJRefresh/MJRefreshConst.m

@@ -16,7 +16,7 @@
 NSString *const MJRefreshFooterReleaseToRefresh = @"松开立即加载更多数据";
 NSString *const MJRefreshFooterRefreshing = @"MJ哥正在帮你加载数据...";
 
-NSString *const MJRefreshHeaderPullToRefresh = @"上拉可以刷新";
+NSString *const MJRefreshHeaderPullToRefresh = @"下拉可以刷新";
 NSString *const MJRefreshHeaderReleaseToRefresh = @"松开立即刷新";
 NSString *const MJRefreshHeaderRefreshing = @"MJ哥正在帮你刷新...";
 NSString *const MJRefreshHeaderTimeKey = @"MJRefreshHeaderView";