Use regular asciidoctor-pdf

The --pre pulled in version 2.0.0.alpha.1 which has an
incompatibility with Ruby psych

    # /usr/lib64/ruby/gems/2.5.0/gems/asciidoctor-pdf-2.0.0.alpha.1/lib/asciidoctor/pdf/theme_loader.rb line 101
    yaml_data = ::YAML.safe_load data, aliases: true, filename: filename

    # /usr/lib64/ruby/2.5.0/psych.rb line 313
    def self.safe_load yaml, whitelist_classes = [], whitelist_symbols = [], aliases = false, filename = nil, symbolize_names: false

Stacktrace:
    1: from /usr/lib64/ruby/gems/2.5.0/gems/asciidoctor-pdf-2.0.0.alpha.1/lib/asciidoctor/pdf/theme_loader.rb:101:in `load_file'
    /usr/lib64/ruby/2.5.0/psych.rb:313:in `safe_load': unknown keywords: aliases, filename (ArgumentError)

Currently not sure what's wrong with the call.

tools/generate-documentation

@@ -185,7 +185,7 @@ check_asciidoctor() {
     if [[ -z "$asciidoctor_bin" ]] || [[ ! -f $asciidoctor_bin ]]; then
         echo "Could not find asciidoctor binary in your path, please install it and run this command again:"
         echo "    sudo gem install asciidoctor pygments.rb"
-        [[ ${formats[pdf]} ]] && echo "    sudo gem install asciidoctor-pdf --pre"
+        [[ ${formats[pdf]} ]] && echo "    sudo gem install asciidoctor-pdf"
         echo "or via openSUSE's package manager:"
         echo "    sudo zypper install 'rubygem(asciidoctor)' 'perl(Pod::AsciiDoctor)'"
         exit 1
@@ -195,7 +195,7 @@ check_asciidoctor() {
 install_asciidoctor() {
     # install dependencies
     gem install asciidoctor pygments.rb
-    [[ ${formats[pdf]} ]] && gem install asciidoctor-pdf --pre
+    [[ ${formats[pdf]} ]] && gem install asciidoctor-pdf
     cpanm -M https://cpan.metacpan.org --install Pod::AsciiDoctor
 }