Please note the following addition:

Instructions
Stockinette Section
Using yarn A, cast on 7 sts.
Row 1 (RS): K1, yo, k1, yo, k1, yo, k1 and place marker on this st just
worked (this is the central spine st; you will move marker up
on this st every row), yo, k1, yo, k1, yo, k1. (11 sts)

Please note the following addition:

Instructions
Stockinette Section
Using yarn A, cast on 7 sts.
Row 1 (RS): K1, yo, k1, yo, k1, yo, k1 and place marker on this st just
worked (this is the central spine st; you will move marker up
on this st every row), yo, k1, yo, k1, yo, k1. (11 sts)