fix(readme): update link to api.md to use the correct branch (#145)

Author: stainless-bot

README.md

@@ -90,7 +90,7 @@ yarn add @anthropic-ai/sdk
 
 ## Usage
 
-The full API of this library can be found in [api.md](https://www.github.com/anthropics/anthropic-sdk-typescript/blob/master/api.md).
+The full API of this library can be found in [api.md](https://www.github.com/anthropics/anthropic-sdk-typescript/blob/main/api.md).
 
 ```js
 import Anthropic from '@anthropic-ai/sdk';