fix(p4): IS -> IX (#488)

Author: skyzh

src/include/concurrency/lock_manager.h

@@ -149,7 +149,7 @@ class LockManager {
  * A lock request being upgraded should be prioritised over other waiting lock requests on the same resource.
  *
  * While upgrading, only the following transitions should be allowed:
- * IS -> [S, X, SIX]
+ * IS -> [S, X, IX, SIX]
  * S -> [X, SIX]
  * IX -> [X, SIX]
  * SIX -> [X]