[BUG] Signup link is beta.openai.com is no longer active

[jk-codertech]

For a developer to come and read openai-cookbook readme and giving the wrong signup page info is not accepted. Readme still refers to beta.openai.com. It should refer to platform.openai.com

Replace this beta.openai.com with platform.openai.com where ever it is used inside this repo as beta.openai.com is no longer active

[brandonbaker-openai]

@jk-codertech Thanks for bringing this up - Fixed in #1825