[docs] Install whisper dependencies by specifying extra

README.rst

@@ -169,7 +169,7 @@ Whisper (for Whisper users)
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
 Whisper is **required if and only if you want to use whisper** (``recognizer_instance.recognize_whisper``).
 
-You can install it with ``python3 -m pip install openai-whisper soundfile``.
+You can install it with ``python3 -m pip install SpeechRecognition[whisper-local]``.
 
 Whisper API (for Whisper API users) 
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
@@ -178,7 +178,7 @@ The library `openai <https://pypi.org/project/openai/>`__ is **required if and o
 
 If not installed, everything in the library will still work, except calling ``recognizer_instance.recognize_whisper_api`` will raise an ``RequestError``.
 
-You can install it with ``python3 -m pip install openai``.
+You can install it with ``python3 -m pip install SpeechRecognition[whisper-api]``.
 
 Troubleshooting
 ---------------