docs: rbtree.txt: fix Sphinx build warnings

Ths file is already at ReST format. Yet, some recent changes
made it to produce a few warnings when building it with
Sphinx.

Those are trivially fixed by marking some literal blocks.

Fix them before adding it to the docs building system.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

Documentation/rbtree.txt

@@ -204,21 +204,21 @@ potentially expensive tree iterations. This is done at negligible runtime
 overhead for maintanence; albeit larger memory footprint.
 
 Similar to the rb_root structure, cached rbtrees are initialized to be
-empty via:
+empty via::
 
   struct rb_root_cached mytree = RB_ROOT_CACHED;
 
 Cached rbtree is simply a regular rb_root with an extra pointer to cache the
 leftmost node. This allows rb_root_cached to exist wherever rb_root does,
 which permits augmented trees to be supported as well as only a few extra
-interfaces:
+interfaces::
 
   struct rb_node *rb_first_cached(struct rb_root_cached *tree);
   void rb_insert_color_cached(struct rb_node *, struct rb_root_cached *, bool);
   void rb_erase_cached(struct rb_node *node, struct rb_root_cached *);
 
 Both insert and erase calls have their respective counterpart of augmented
-trees:
+trees::
 
   void rb_insert_augmented_cached(struct rb_node *node, struct rb_root_cached *,
 				  bool, struct rb_augment_callbacks *);