docs/library/btree.rst: Correct method typo: __detitem__ to __delitem__.

docs/library/btree.rst

@@ -118,7 +118,7 @@ Methods
 .. method:: btree.__getitem__(key)
             btree.get(key, default=None, /)
             btree.__setitem__(key, val)
-            btree.__detitem__(key)
+            btree.__delitem__(key)
             btree.__contains__(key)
 
    Standard dictionary methods.