docs: use RuleConfigSeverity in config docs (#3521)

There is the following enum for checking rule config severity
in `commitlint/@commitlint/types/src/rules` that we should use
in typescript plugins instead of 0, 1, and 2 numbers to make
the code clearer.

```
export enum RuleConfigSeverity {
	Disabled = 0,
	Warning = 1,
	Error = 2,
}
```

Author: tehraninasab

Diff for: docs/reference-configuration.md

@@ -72,6 +72,7 @@ module.exports = Configuration;
 
 ```ts
 import type {UserConfig} from '@commitlint/types';
+import { RuleConfigSeverity } from "@commitlint/types";
 
 const Configuration: UserConfig = {
   /*
@@ -93,7 +94,7 @@ const Configuration: UserConfig = {
    * Any rules defined here will override rules from @commitlint/config-conventional
    */
   rules: {
-    'type-enum': [2, 'always', ['foo']],
+    'type-enum': [RuleConfigSeverity.Error, 'always', ['foo']],
   },
   /*
    * Functions that return true if commitlint should ignore the given message.