Bug fix bin/output-year-index.sh (unset variable)

The PANDOC_WRAPPER did not have a default value which not only caused
the help string to show something like: '(def: use )' but it meant one
had to specify the option -p to specify the pandoc wrapper tool (which
is bin/pandoc-wrapper.sh).

bin/output-year-index.sh

@@ -127,6 +127,7 @@ export TOPDIR
 export DOCROOT_SLASH="../../"
 export REPO_URL="https://github.com/ioccc-src/temp-test-ioccc/blob/master"
 export SITE_URL="https://ioccc-src.github.io/temp-test-ioccc"
+export PANDOC_WRAPPER="bin/pandoc-wrapper.sh"
 
 # set usage message
 #