Fix types of rule arguments

Closes GH-289.

Reviewed-by: Titus Wormer <tituswormer@gmail.com>

Author: jablko

Diff for: packages/unified-lint-rule/index.d.ts

@@ -19,7 +19,7 @@ export function lintRule<Tree extends Node = Node, Options = unknown>(
   name: string | RuleMeta,
   rule: Rule<Tree, Options>
 ): Plugin<
-  void[] | [Options | Label | Severity] | [boolean | Label | Severity, Options],
+  void[] | [Options | [boolean | Label | Severity, (Options | undefined)?]],
   Tree
 >
 

Diff for: readme.md

@@ -269,7 +269,8 @@ remark()
   .use(remarkLintUnorderedListMarkerStyle, '*')
   .use(remarkLintUnorderedListMarkerStyle, ['on', '*'])
   .use(remarkLintUnorderedListMarkerStyle, [1, '*'])
-  // The following rule accepts a number, numbers *must* be passed in arrays:
+  // The following rule accepts a number:
+  .use(remarkLintMaximumLineLength, 72)
   .use(remarkLintMaximumLineLength, ['on', 72])
   .use(remarkLintMaximumLineLength, [1, 72])
 ```