Sender should be able to update its own memos

[netnose]

Description

In BuyMeACoffee contract a sender couldn't update its own memos, it should de able to do it

Additional Information

• I have read the contributing docs (if this is your first contribution)
• This is not a duplicate of any existing pull request

Your ENS/address: netnose.eth

[netnose]

@winnsterx

[tienshaoku]

I'd also suggest extracting this condition from both modifyMemoMessage() & removeMemo() to avoid typo like this!
However, I'm unsure about whether extracting logic is the expected complexity in this tutorial, so lmk and I'm happy to help commit this change 😁

[Alann93]

Me transfieres las monedas a 0xee55fb66f259ec5d5386459f17dd415becb9cdc3