DOC: Update API key length

Author: cjdsellers

README.md

@@ -45,8 +45,8 @@ To install the latest stable version of the package from PyPI:
 ## Usage
 The library needs to be configured with an API key from your account.
 [Sign up](https://databento.com/signup) for free and you will automatically
-receive a set of API keys to start with. Each API key is a 28-character
-string that can be found on the API Keys page of your [Databento user portal](https://databento.com/platform/keys).
+receive a set of API keys to start with. Each API key is a 32-character
+string starting with `db-`, that can be found on the API Keys page of your [Databento user portal](https://databento.com/platform/keys).
 
 A simple Databento application looks like this: