Update description about jsBundleURLForBundleRoot method (#39265)

Summary:
Update comment in RCTBundleURLProvider.h to use correct `jsBundleURLForBundleRoot` import with `fallbackExtension` instead `fallbackResource`

## Changelog:

- Update comment in `jsBundleURLForBundleRoot` definition on RCTBundleURLProvider.h file.

Pick one each for the category and type tags:

[IOS][CHANGED] - Updated comment in RCTBundleURLProvider.h to instance correct jsBundleURLForBundleRoot:fallbackExtension method.

Pull Request resolved: #39265

Test Plan: N/A

Reviewed By: cortinico

Differential Revision: D49227104

Pulled By: GijsWeterings

fbshipit-source-id: 6ccfb984ab6c1e858f1c568b2dd8f4626e133ffb

packages/react-native/React/Base/RCTBundleURLProvider.h

@@ -112,7 +112,7 @@ RCT_EXTERN void RCTBundleURLProviderAllowPackagerServerAccess(BOOL allowed);
 
 /**
  * Given a hostname for the packager and a bundle root, returns the URL to the js bundle. Generally you should use the
- * instance method -jsBundleURLForBundleRoot:fallbackResource: which includes logic to guess if the packager is running
+ * instance method -jsBundleURLForBundleRoot:fallbackExtension: which includes logic to guess if the packager is running
  * and fall back to a pre-packaged bundle if it is not.
  *
  * The options here mirror some of Metro's Bundling Options: