docs: update extending-matchers.md (#3557)

Co-authored-by: Vladimir <sleuths.slews0s@icloud.com>

docs/guide/extending-matchers.md

@@ -26,6 +26,8 @@ expect.extend({
 If you are using TypeScript, since Vitest 0.31.0 you can extend default `Assertion` interface in an ambient declaration file (e.g: `vitest.d.ts`) with the code below:
 
 ```ts
+import type { Assertion, AsymmetricMatchersContaining } from 'vitest'
+
 interface CustomMatchers<R = unknown> {
   toBeFoo(): R
 }