[skip ci] Add prompt usage in README

Author: KitaitiMakoto

bindings/ruby/README.md

@@ -31,6 +31,7 @@ params.duration = 60_000
 params.max_text_tokens = 300
 params.translate = true
 params.print_timestamps = false
+params.prompt = "John F. Kennedy says: "
 
 whisper.transcribe("path/to/audio.wav", params) do |whole_text|
   puts whole_text