Yes, it follows from:
In order to sort A[1..n], we recursively sort A[1..n-1] and then insert A[n] into the sorted array A[1..n-1]
The "recursively sort A[1..n-1]" part takes T(n-1) time (this is easy: we're defining T(n) to mean "the time it takes to sort n elements", so the time it takes to sort n-1 elements is trivially T(n-1)), while the "insert A[n] into the sorted array A[1..n-1]" part takes (worst case) O(n) time. Add them together to get
T(n) = T(n-1) + O(n)