I noticed something interesting. In the translation one Hebrew phrase is transliterated into English rather than translated. But obviously "Roshei Yeshiva" is not exactly a transliteration of "ראשי ישיבות." It is surprising that Artscroll not only chose to make a grammatical correction but to do it so visibly.
Click the image below to enlarge:
(If anyone is curious, in the Otzar Ha-chochma "roshei yeshiva" returned 795 results, "roshei yeshivos" returned 2890 (both spelled in Hebrew, of course). I'd say this makes a decent case that "roshei yeshivos" is acceptable rabbinic Hebrew. Included in the "roshei yeshivos" users were people like R. Yaakov Emden, and Italian rabbis like R. Samson Morpurgo and R. Aviad Sar Shalom Basilea, who knew about grammar.
For comparison purposes I also searched hebrewbooks.org and the Bar Ilan Responsa database, which essentially confirmed that "roshei yeshivos" is, descriptively speaking, rabbinic Hebrew. Spelling it in English, or in Hebrew, Google returns results for "roshei yeshiva" by the tens of thousands more than "roshei yeshivos.")