Update formats.md doc for new allowed extensions.

This PR documents the changes made in https://github.com/gohugoio/hugo/pull/7447/files adding two other extensions to the Allowed list.

docs/content/en/content-management/formats.md

@@ -75,7 +75,7 @@ Parameter | Default | Comment
 --- | --- | --- 
 backend | `html5` | Don't change this unless you know what you are doing.
 doctype | `article` | Document type (article, book or manpage).
-extensions | | Possible extensions are `asciidoctor-html5s`, `asciidoctor-diagram`,	`asciidoctor-interdoc-reftext`, `asciidoctor-katex`, `asciidoctor-latex`, `asciidoctor-question`, `asciidoctor-rouge`.
+extensions | | Possible extensions are `asciidoctor-html5s`, `asciidoctor-bibtex`, `asciidoctor-diagram`,	`asciidoctor-interdoc-reftext`, `asciidoctor-katex`, `asciidoctor-latex`, `asciidoctor-mathematical`, `asciidoctor-question`, `asciidoctor-rouge`.
 attributes | | Variables to be referenced in your `adoc` file. This is a list of variable name/value maps. See [Asciidoctor#attributes](https://asciidoctor.org/docs/asciidoc-syntax-quick-reference/#attributes-and-substitutions).
 noheaderorfooter | true | Output an embeddable document, which excludes the header, the footer, and everything outside the body of the document. Don't change this unless you know what you are doing. 
 safemode | `unsafe` | Safe mode level `unsafe`, `safe`, `server` or `secure`. Don't change this unless you know what you are doing.