Fixes cli error for Asciidoctor (#12677)

The original fix for #8676 introduced illegal arguments for Asciidoctor, causing no rendering at all. This PR fixes the command line arguments so that Asciidoctor properly renders the text. See https://asciidoctor.org/docs/user-manual/#piping-content-through-the-cli as reference.

docs/content/doc/advanced/external-renderers.en-us.md

@@ -53,7 +53,7 @@ add one `[markup.XXXXX]` section per external renderer on your custom `app.ini`:
 [markup.asciidoc]
 ENABLED = true
 FILE_EXTENSIONS = .adoc,.asciidoc
-RENDER_COMMAND = "asciidoctor -e -a leveloffset=-1 --out-file=- -"
+RENDER_COMMAND = "asciidoctor -s -a showtitle --out-file=- -"
 ; Input is not a standard input but a file
 IS_INPUT_FILE = false
 
@@ -93,4 +93,4 @@ To define multiple entries, add a unique alphanumeric suffix (e.g., `[markup.san
 Once your configuration changes have been made, restart Gitea to have changes take effect.
 
 **Note**: Prior to Gitea 1.12 there was a single `markup.sanitiser` section with keys that were redefined for multiple rules, however,
-there were significant problems with this method of configuration necessitating configuration through multiple sections.
+there were significant problems with this method of configuration necessitating configuration through multiple sections.