doc: first letter should be upper case

src/libcollections/binary_heap.rs

@@ -173,7 +173,7 @@
 /// ```
 /// use std::collections::BinaryHeap;
 ///
-/// // type inference lets us omit an explicit type signature (which
+/// // Type inference lets us omit an explicit type signature (which
 /// // would be `BinaryHeap<i32>` in this example).
 /// let mut heap = BinaryHeap::new();
 ///