doc: reword custom indices section #2

doc/startify.txt

@@ -148,10 +148,9 @@ Example: let g:startify_skiplist_server = [ 'GVIM' ]
 
     let g:startify_custom_indices = []
 
-Use any list of strings as custom indices instead of increasing numbers. If
-there are more paths to fill in the startify buffer than actual items in the
-custom list, the rest will be filled up using the default numbering scheme
-starting from 0.
+Use any list of strings as indices instead of increasing numbers. If there are
+more startify entries than actual items in the custom list, the remaining
+entries will be filled using the default numbering scheme starting from 0.
 
 Example: let g:startify_custom_indices = ['a','s','d','f']