[BUG] claude code beta header docs is wrong

Environment

• Platform (select one):
• [ ] Anthropic API
• [ ] AWS Bedrock
• [x] Google Vertex AI
• [x] Other: <!-- specify -->
• Claude CLI version: <!-- output of claude --version -->
• Operating System: <!-- e.g. macOS 14.3, Windows 11, Ubuntu 22.04 -->
• Terminal: <!-- e.g. iTerm2, Terminal App -->

Bug Description

in docs, it says ANTHROPIC_CUSTOM_HEADERS env var is used for configuring beta flags. but from source code (claude code v1.0.107) it uses ANTHROPIC_BETA, ANTHROPIC_BETAS instead)

<img width="801" height="483" alt="Image" src="https://github.com/user-attachments/assets/f64be0e6-09c5-45a5-89a8-d9f2e3da9458" />

<img width="1169" height="104" alt="Image" src="https://github.com/user-attachments/assets/a19be87e-56ca-414e-b504-cd1dbaeb970c" />