Update the default free memory pointer in Yul.rst

The solidity docs and [Inline assembly memory management](https://docs.soliditylang.org/en/v0.8.15/assembly.html#memory-management) suggest the actual allocate-able memory starts from `0x80`. The above yul example defaults the free memory pointer to `0x60` in initialisation cases.

docs/yul.rst

@@ -1162,7 +1162,7 @@ An example Yul Object is shown below:
         code {
             function allocate(size) -> ptr {
                 ptr := mload(0x40)
-                if iszero(ptr) { ptr := 0x60 }
+                if iszero(ptr) { ptr := 0x80 }
                 mstore(0x40, add(ptr, size))
             }