It's not an accent (those are usually lines) but a diaeresis (two dots).
It separates the two vowels.
A bit like in older English spelling coöperate which is pronounced "co-op-er-ate" and not "coop-er-ate" (the two o's are pronounced separately rather than as an oo sound), or naïve which does not rhyme with waive, the diaeresis in προϊστάμενος shows that it's pro-is-ta-me-nos and not pris-ta-me-nos (since οι makes the /i/ sound).
I was under the (apparent) misapprehension that "administrator" would also have been a viable alternative to "supervisor". In a legal office setting it would have seemed perhaps more logical, but there again not everything on DL follows that kind of logic, perhaps designed to trip us up/ think outside our normal boxes! ;-)}