[libc][newheadergen]: PyYaml Version Update (#100463)

Summary:
- a lot of builds had an issue using new headergen because they do not
have PyYaml installed.

Test Plan: 

Reviewers: 

Subscribers: 

Tasks: 

Tags: 


Differential Revision: https://phabricator.intern.facebook.com/D60250677

Author: aaryanshukla

libc/docs/dev/header_generation.rst

@@ -22,7 +22,8 @@ Instructions
 ------------
 
 Required Versions:
-  - Python Version: 3.11.8
+  - Python Version: 3.6
+  - PyYAML Version: 5.1
 
 1. Keep full-build mode on when building, otherwise headers will not be
    generated.