doc: CI badge should point to main instead of master

Point to the new branch to get the correct status.

Signed-off-by: Anas Nashif <[email protected]>

Author: nashif

README.rst

@@ -10,7 +10,7 @@
    src="https://bestpractices.coreinfrastructure.org/projects/74/badge"></a>
    <a href="https://buildkite.com/zephyr/zephyr">
    <img
-   src="https://badge.buildkite.com/f5bd0dc88306cee17c9b38e78d11bb74a6291e3f40e7d13f31.svg?branch=master"></a>
+   src="https://badge.buildkite.com/f5bd0dc88306cee17c9b38e78d11bb74a6291e3f40e7d13f31.svg?branch=main"></a>
 
 
 The Zephyr Project is a scalable real-time operating system (RTOS) supporting