I have been using the following code on Apps Script on google sheets which successfully adds a timestamp to column A whenever any cell on that row is edited.

I copied the code from point 4 of this wikihow article and just changed M to A
(I can’t copy and paste it in here due to the formatting)
(https://www.wikihow.com/Google-Sheets-How-to-Insert-Time-in-Cell-Automatically)

How do I change this code so that it only adds the timetstamp if the cell in column B on that row is edited? I don’t know much about this stuff and can’t work out how to effectively change the range.