Fix typo(?)

This potential typo originates from
<jercaianu@b0f461b#diff-16009a9dfdb03aeb4f92c4160f8393cb875bf0398260b9c27e57c1219df5b90eR792>
which was the original PR.

But I don't think this is right.
D-Scanner is unhappy about it at least.
And I assume this functions just lacks test coverage.

std/experimental/allocator/building_blocks/allocator_list.d

@@ -791,7 +791,7 @@ public:
         lock.lock();
         scope(exit) lock.unlock();
 
-        return assumeUnshared(impl).reallocate(b, delta);
+        return assumeUnshared(impl).reallocate(b, s);
     }
 
     /**