Fix stack layout comment in GET_SLOT_FROM_KEYS_2D (#90)

In the `concat the two keys` step of GET_SLOT_FROM_KEYS_2D (Hashmap.huff), one of the stack layout comments was missing `slot1` before that value is removed from the stack. Added it in the comment.

Author: goncaloMagalhaes

src/data-structures/Hashmap.huff

@@ -49,7 +49,7 @@
     sha3                // [slot1, key2]
 
     // concat the two keys
-    <mem_ptr> 0x20 add  // [<mem_ptr> + 0x20, key2] put slot1 in memory
+    <mem_ptr> 0x20 add  // [<mem_ptr> + 0x20, slot1, key2] put slot1 in memory
     mstore              // [key2]
 
     // next byte
@@ -102,4 +102,4 @@
     // Input stack: [slot, key1, key2, value]
     GET_SLOT_FROM_KEYS_2D(<mem_ptr>)    // [slot, value]
     sstore                              // []
-}
+}