While a misspelled middle name would not automatically invalidate the document, Standard Legal strongly suggests that the document be corrected so that no issues might arise in the future.
Some person or entity would need to challenge the legitimacy of the original document with respect to that error, which is not likely — but the error certainly leaves open the possibility of such a challenge.
You may contact the notary to see if this simple change may be allowed, but this circumstance may require creating a brand new document and having it notarized again.
Details on creating a new Power of Attorney document through Standard Legal’s legal forms software is found here.