docs: update npm init @eslint/config generated file names (#18298)

* docs: update `eslint init @eslint/config` generated file names

Signed-off-by: 唯然 <[email protected]>

* Update getting-started.md

---------

Signed-off-by: 唯然 <[email protected]>

README.md

@@ -59,7 +59,7 @@ After that, you can run ESLint on any file or directory like this:
 
 ## Configuration
 
-After running `npm init @eslint/config`, you'll have an `eslint.config.js` or `eslint.config.mjs` file in your directory. In it, you'll see some rules configured like this:
+After running `npm init @eslint/config`, you'll have an `eslint.config.js` (or `eslint.config.mjs`) file in your directory. In it, you'll see some rules configured like this:
 
 ```js
 import pluginJs from "@eslint/js";

docs/src/use/getting-started.md

@@ -50,7 +50,7 @@ yarn run eslint yourfile.js
 
 **Note:** If you are coming from a version before 9.0.0 please see the [migration guide](configure/migration-guide).
 
-After running `npm init @eslint/config`, you'll have an `eslint.config.js` file in your directory. In it, you'll see some rules configured like this:
+After running `npm init @eslint/config`, you'll have an `eslint.config.js` (or `eslint.config.mjs`) file in your directory. In it, you'll see some rules configured like this:
 
 ```js
 // eslint.config.js