doc: fix typo in comment explaining rb_tree usage

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

  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

  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

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

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

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

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

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

  not trivial work to rebalance the rbtree if necessary.

  rbtree, if necessary.

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

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