latex: fix signal names to standardized (pelican-3.2 and later)

The names were accidentally reverted to pre-pelican-3.2 in
5fa7ddc0ea08d517c4a767ceaa1df6ccc4d721c "revised the latex plugin so
that it will work in both http and https protocols".
Merge pull request #93 from jedbrown/jed/fix-latex-signal-name

latex: fix signal names to standardized (pelican-3.2 and later)