Is making minor adjustments to an online tutorial for a coding assignment considered plagiarism?
General Rule: Make it clear what your own contributions are, and where the other stuff is coming from.

Simply adding an explanation to your coursework on how you followed the tutorial, how far your modifications went, etc makes sure that you are not committing plagiarism. However, it is still perfectly plausible that your lecturer decides that the extent of your own contribution is insufficient. Thus, I would recommend checking in advance of the deadline.