Yup, you would want to play the B as the first note for that measure, thats what the composer is asking for. It's helpful to know that the chord is a G chord and not a B chord because that tells you what other notes are diatonically available (G, B, D). If the chord were a B chord, the chord would be structured using the notes B, D#, F#. So given that this is a G/B chord, it tells you that you should use notes in the G chord but begin with B, if you chose to add more notes, other than B, to your bass line. Hope this helps!