Just add <br>
where you would like to make the new line.
$S$: a set of shops
<br>
$I$: a set of items M wants to get
Because jupyter notebook markdown cell is a superset of HTML.
http://jupyter-notebook.readthedocs.io/en/latest/examples/Notebook/Working%20With%20Markdown%20Cells.html
Note that newlines using <br>
does not persist when exporting or saving the notebook to a pdf (using "Download as > PDF via LaTeX"). It is probably treating each <br>
as a space.