"Probable reading," i.e. how it was probably meant to be read. There are issues of translation which crop up, and Safire in this case doesn't have room to go into full detail about possible readings and details of the translation. As Bruce put it in History of the Bible in English:

It is not always easy to determine whether we are being offered a conjectural emendation or a conjectural reinterprettion, except that the former is normally indicated by a footnote saying "prob rdg." (i.e. "probable reading") followed by a remark that the Hebrew is obscure, or corrupt, or omits something given in the new version, or adds something not given in the version, or the like.