Fix check for asciidoctor gem
The previous check was opposite to what we wanted.
# Make sure the user has installed the asciidoc gem, otherwise the website will
# generate successfully but it will be missing tocs, titles, and other
# attributes.
-if gem list -i asciidoctor &>/dev/null; then
+if ! gem list -i asciidoctor &> /dev/null; then
echo "You don't have the asciidoctor gem installed."
echo "Please run 'gem install --user asciidoctor' before executing this script."
exit 1
