docs/library/btree: Fix method links to explicitly specify class.

So they don't clash with other potential references.

Signed-off-by: Koudai Aono <[email protected]>

Author: koxudaxi

docs/library/btree.rst

@@ -151,10 +151,10 @@ Constants
 
 .. data:: INCL
 
-   A flag for `keys()`, `values()`, `items()` methods to specify that
+   A flag for :meth:`btree.keys`, :meth:`btree.values`, :meth:`btree.items` methods to specify that
    scanning should be inclusive of the end key.
 
 .. data:: DESC
 
-   A flag for `keys()`, `values()`, `items()` methods to specify that
+   A flag for :meth:`btree.keys`, :meth:`btree.values`, :meth:`btree.items` methods to specify that
    scanning should be in descending direction of keys.