use consistent terminology in CLI usage for option that may be specif…

…ied more than once

lib/asciidoctor/cli/options.rb

@@ -94,7 +94,7 @@ def parse!(args)
             self[:attributes][name] = val
           end
           opts.on('-T', '--template-dir DIR', 'a directory containing custom converter templates that override the built-in converter (requires tilt gem)',
-              'may be specified multiple times') do |template_dir|
+              'may be specified more than once') do |template_dir|
             if self[:template_dirs].nil?
               self[:template_dirs] = [template_dir]
             elsif ::Array === self[:template_dirs]