Fix the README file for MERGE command

Hi Hackers,

This is Myo Wai Thant.

I found out that there is a mistake written in executor/README file.

The actions of MERGE command can be specified as follows: INSERT, UPDATE, DELETE and DO NOTHING.

However, in the README file, the ‘UPDATE’ word is described 2 times instead of ‘DELETE’.

Therefore, I attached the patch file which fix this word usage.

It would be great if you could take a look at it.

Thank you.

Best Regards,

Myo Wai Thant