[IMPR]: clarify variable name in djvutext.py
[IMPR]: clarify variable name in djvutext.py

Clarify that we are working on a page interval, not a page.

Change-Id: Idf2fb56ecae4ad975f03d048244e5f141fb497a9

wikimedia/pywikibot-core,wikimedia/pywikibot-core