[libc++] Fix some one-off typos in comments. NFCI.

GitOrigin-RevId: b6f191741562ab32a3e210c8661df09b546275cc
diff --git a/include/__tree b/include/__tree
index 26c4c41..d1bfccf 100644
--- a/include/__tree
+++ b/include/__tree
@@ -1428,7 +1428,7 @@
     __node_base_pointer&
         __find_leaf(const_iterator __hint,
                     __parent_pointer& __parent, const key_type& __v);
-    // FIXME: Make this function const qualified. Unfortunetly doing so
+    // FIXME: Make this function const qualified. Unfortunately doing so
     // breaks existing code which uses non-const callable comparators.
     template <class _Key>
     __node_base_pointer&