chore: fix typo in hash code for Expr equality test (leanprover#4990)

We observed a small performance improvement at
https://github.com/leanprover/LNSym/blob/proof_size_expt/Proofs/SHA512/Experiments/Sym30.lean
Before: 2.65s
After: 2.60s

Author: leodemoura

src/kernel/expr_eq_fn.cpp

@@ -23,7 +23,7 @@ template<bool CompareBinderInfo>
 class expr_eq_fn {
     struct key_hasher {
         std::size_t operator()(std::pair<lean_object *, lean_object *> const & p) const {
-            return hash((size_t)p.first >> 3, (size_t)p.first >> 3);
+            return hash((size_t)p.first >> 3, (size_t)p.second >> 3);
         }
     };
     typedef std::unordered_set<std::pair<lean_object *, lean_object *>, key_hasher> cache;