fix a couple of function calls that should have been renamed

Call nerdtree#echo, not s:echo. This was changed in the big refactor.

Fixes preservim#224.

Author: scrooloose

plugin/nerdtree/bookmark.vim

@@ -290,7 +290,7 @@ function! s:Bookmark.validate()
     else
         call s:Bookmark.CacheBookmarks(1)
         call nerdtree#renderView()
-        call s:echo(self.name . "now points to an invalid location. See :help NERDTreeInvalidBookmarks for info.")
+        call nerdtree#echo(self.name . "now points to an invalid location. See :help NERDTreeInvalidBookmarks for info.")
         return 0
     endif
 endfunction

plugin/nerdtree/path.vim

@@ -68,7 +68,7 @@ function! s:Path.changeToDir()
 
     try
         execute "cd " . dir
-        call s:echo("CWD is now: " . getcwd())
+        call nerdtree#echo("CWD is now: " . getcwd())
     catch
         throw "NERDTree.PathChangeError: cannot change CWD to " . dir
     endtry