Fix @react-native/oss-library-example package.json#exports (#44179)

Summary:
Pull Request resolved: #44179

Fix `package.json#exports` main entry point in `react-native/oss-library-example` (which is actually at `packages/react-native-test-library`), to fix a Metro resolver warning on building RN-tester.

`"./": "./index.js"` is intended as an export map entry for the main export, whereas `".": "./index.js"` is correct (see [`PACKAGE_EXPORTS_RESOLVE`](https://nodejs.org/api/esm.html) spec).

Changelog: [Internal]

(This package is not published)

Reviewed By: cortinico, dmytrorykun

Differential Revision: D56414480

fbshipit-source-id: 01874cf11ae687aaf5aa5aa56075232f03d691b8

Author: robhogan

packages/react-native-test-library/package.json

@@ -12,7 +12,7 @@
   },
   "main": "./index.js",
   "exports": {
-    "./": "./index.js",
+    ".": "./index.js",
     "./package.json": "./package.json"
   },
   "scripts": {