Update README with fingerprint error handling & FirebaseHttpClient.h for US servers

[supunsathsara]

This PR addresses two important updates:

1. README.md Update: Added instructions for handling fingerprint errors in the README.md file. Users encountering fingerprint errors will now be guided on how to update the fingerprint in FirebaseHttpClient.h.

2. FirebaseHttpClient.h Update: Updated the FirebaseHttpClient.h file to reflect changes for US servers as of February 2024. This ensures that the library connects properly to the Firebase servers based in the US.

These updates enhance the usability and reliability of the FirebaseArduino library.

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[supunsathsara]

For others looking to fix fingerprint error,
If you encounter a fingerprint error while using the FirebaseArduino library, you can follow these steps to update the fingerprint:

1. Navigate to the FirebaseHttpClient.h file located at Documents > Arduino > libraries > FirebaseArduino > src > FirebaseHttpClient.h.

2. Find the section in the file where the fingerprint is defined.

3. Visit this link to obtain the latest fingerprint for the Firebase servers.

4. Replace the existing fingerprint in the FirebaseHttpClient.h file with the new one.

5. Save the file and recompile your project.

This should resolve the fingerprint error and allow your project to connect to the Firebase servers successfully. If you have any further questions or issues, feel free to ask for assistance.