Code typos on `content/07a_opcodes /09_memory.ipynb`

[0x6980]

In the function mstore(evm)

def mstore(evm): 
    # TODO: should be right aligned
    offset, value = evm.stack.pop(), cpu.stack.pop()
    evm.memory.store(offset, value)
    evm.pc += 1

cpu.stack.pop() should be evm.stack.pop()