Remove unneeded asterisk

Related-to: GH-2503.

docs/docs/using-mdx.mdx

@@ -309,7 +309,7 @@ The following keys can be passed in `components`:
 * HTML equivalents for the things you write with markdown such as `h1` for
   `# heading` (see [§ Table of components][toc] for examples)
 * `wrapper`, which defines the layout (but a local layout takes precedence)
-* `*` anything else that is a valid JavaScript identifier (`foo`,
+* anything else that is a valid JavaScript identifier (`foo`,
   `Quote`, `_`, `$x`, `a1`) for the things you write with JSX (like
   `<So />` or `<like.so />`, note that locally defined components take
   precedence)**‡**