doc: fix typo in comment explaining rb_tree usage

  Some example of insert and search follows here. The search is a plain

  normal search over an ordered tree. The insert instead must be implemented

  int two steps: as first thing the code must insert the element in

  order as a red leaf in the tree, then the support library function

  rb_insert_color() must be called. Such function will do the

  not trivial work to rebalance the rbtree if necessary.

  in two steps: First, the code must insert the element in order as a red leaf

  in the tree, and then the support library function rb_insert_color() must

  be called. Such function will do the not trivial work to rebalance the

  rbtree, if necessary.

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static inline struct page * rb_search_page_cache(struct inode * inode,