fix: update error message to use npx when calling react-native (#42691)

Summary:
Just small fix for those who are copy & pasting commands from error message.

## Changelog:

[INTERNAL] [CHANGED] - Update erorr message to use `npx` when calling `react-native`

Pull Request resolved: https://github.com/facebook/react-native/pull/42691

Test Plan: _

Reviewed By: cipolleschi

Differential Revision: D53177186

Pulled By: cortinico

fbshipit-source-id: e680cde81fde1f560dfeb1a85c8ad90090d69653

packages/react-native/Libraries/Core/ReactNativeVersionCheck.js

@@ -34,7 +34,7 @@ const checkVersions = function checkVersions(): void {
         `Native version: ${_formatVersion(nativeVersion)}\n\n` +
         'Make sure that you have rebuilt the native code. If the problem ' +
         'persists try clearing the Watchman and packager caches with ' +
-        '`watchman watch-del-all && react-native start --reset-cache`.',
+        '`watchman watch-del-all && npx react-native start --reset-cache`.',
     );
   }
 };