#2647 two link error for html/ made by "make html"

[Shigeharu TAKENO]

I found two link error for html/ make by "make html".

(1) The file "figure_mask.png" is not found in html/, but it is referred by loc5723.html. The link is made by the ^ line in "2 masking" of gnuplot.doc.

(2) Terminals.html (= loc18703.html) exists for "1 Terminal types" section, but loc18705.html doesn't exist for "2 complete list of terminals", which is referred by index.html and loc16036.html for "?terminal", "?term", and "?complete list of terminals" lines.