Doc correction: btree

Author: athulappadan

src/libcollections/btree/set.rs

@@ -674,7 +674,7 @@ impl<'a, T: 'a + Ord + Copy> Extend<&'a T> for BTreeSet<T> {
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: Ord> Default for BTreeSet<T> {
-    /// Makes a empty `BTreeSet<T>` with a reasonable choice of B.
+    /// Makes an empty `BTreeSet<T>` with a reasonable choice of B.
     fn default() -> BTreeSet<T> {
         BTreeSet::new()
     }