Use the correct scope for the "i" and "s" commands

The docs say that these mappings should have the "FileNode" scope
rather than the "Node" scope, which is actually too broad.

This change fixes the following bug:
 1. Open the NERDTree
 2. Navigate cursor to a directory node with children
 3. Press "i" or "s", and then press "r"
 4. Observe that the node will close when it should remain open

This is due to the usage of an incorrect mapping scope.

autoload/nerdtree/ui_glue.vim

@@ -20,8 +20,8 @@ function! nerdtree#ui_glue#createDefaultBindings()
     call NERDTreeAddKeyMap({ 'key': g:NERDTreeMapActivateNode, 'scope': "Bookmark", 'callback': s."activateBookmark" })
     call NERDTreeAddKeyMap({ 'key': g:NERDTreeMapActivateNode, 'scope': "all", 'callback': s."activateAll" })
 
-    call NERDTreeAddKeyMap({ 'key': g:NERDTreeMapOpenSplit, 'scope': "Node", 'callback': s."openHSplit" })
-    call NERDTreeAddKeyMap({ 'key': g:NERDTreeMapOpenVSplit, 'scope': "Node", 'callback': s."openVSplit" })
+    call NERDTreeAddKeyMap({ 'key': g:NERDTreeMapOpenSplit, 'scope': 'FileNode', 'callback': s.'openHSplit' })
+    call NERDTreeAddKeyMap({ 'key': g:NERDTreeMapOpenVSplit, 'scope': 'FileNode', 'callback': s.'openVSplit' })
 
     call NERDTreeAddKeyMap({ 'key': g:NERDTreeMapPreview, 'scope': "Node", 'callback': s."previewNodeCurrent" })
     call NERDTreeAddKeyMap({ 'key': g:NERDTreeMapPreviewVSplit, 'scope': "Node", 'callback': s."previewNodeVSplit" })