Twenty Fifteen: Add print section in the table of contents in the stylesheet.

[iamtakashi]

Minor but I forgot to add the print section in the table contents in the stylesheet.

[lancewillett]

Good catch.

[SergeyBiryukov]

In 31175:

Twenty Fifteen: Add print section to the table of contents in the stylesheet.

props iamtakashi.
fixes #30993.