Update 10-testing.mdx (#54107)
Fixed `next/jest.js` import in documentation. without `.js` it show this error
```
Error [ERR_MODULE_NOT_FOUND]: Cannot find module 'F:\React\my-project\node_modules\next\jest' imported from F:\React\my-project\jest.config.mjs
Did you mean to import next/jest.js?
at new NodeError (node:internal/errors:399:5)
at finalizeResolution (node:internal/modules/esm/resolve:326:11)
at moduleResolve (node:internal/modules/esm/resolve:945:10)
at defaultResolve (node:internal/modules/esm/resolve:1153:11)
at nextResolve (node:internal/modules/esm/loader:163:28)
at ESMLoader.resolve (node:internal/modules/esm/loader:838:30)
at ESMLoader.getModuleJob (node:internal/modules/esm/loader:424:18)
at ModuleWrap.<anonymous> (node:internal/modules/esm/module_job:77:40)
at link (node:internal/modules/esm/module_job:76:36)
```
